MDADM(8) System Manager's Manual MDADM(8)
mdadm - manage MD devices aka Linux Software RAID
mdadm [mode] <raiddevice> [options] <component-devices>
RAID devices are virtual devices created from two or more real block
devices. This allows multiple devices (typically disk drives or
partitions thereof) to be combined into a single device to hold (for
example) a single filesystem. Some RAID levels include redundancy
and so can survive some degree of device failure.
Linux Software RAID devices are implemented through the md (Multiple
Devices) device driver.
Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1
(mirroring), RAID4, RAID5, RAID6, RAID10, MULTIPATH, FAULTY, and
CONTAINER.
MULTIPATH is not a Software RAID mechanism, but does involve multiple
devices: each device is a path to one common physical storage device.
New installations should not use md/multipath as it is not well
supported and has no ongoing development. Use the Device Mapper
based multipath-tools instead.
FAULTY is also not true RAID, and it only involves one device. It
provides a layer over a true device that can be used to inject
faults.
CONTAINER is different again. A CONTAINER is a collection of devices
that are managed as a set. This is similar to the set of devices
connected to a hardware RAID controller. The set of devices may
contain a number of different RAID arrays each utilising some (or
all) of the blocks from a number of the devices in the set. For
example, two devices in a 5-device set might form a RAID1 using the
whole devices. The remaining three might have a RAID5 over the first
half of each device, and a RAID0 over the second half.
With a CONTAINER, there is one set of metadata that describes all of
the arrays in the container. So when mdadm creates a CONTAINER
device, the device just represents the metadata. Other normal arrays
(RAID1 etc) can be created inside the container.
mdadm has several major modes of operation:
Assemble
Assemble the components of a previously created array into an
active array. Components can be explicitly given or can be
searched for. mdadm checks that the components do form a bona
fide array, and can, on request, fiddle superblock information
so as to assemble a faulty array.
Build Build an array that doesn't have per-device metadata
(superblocks). For these sorts of arrays, mdadm cannot
differentiate between initial creation and subsequent assembly
of an array. It also cannot perform any checks that
appropriate components have been requested. Because of this,
the Build mode should only be used together with a complete
understanding of what you are doing.
Create Create a new array with per-device metadata (superblocks).
Appropriate metadata is written to each device, and then the
array comprising those devices is activated. A 'resync'
process is started to make sure that the array is consistent
(e.g. both sides of a mirror contain the same data) but the
content of the device is left otherwise untouched. The array
can be used as soon as it has been created. There is no need
to wait for the initial resync to finish.
Follow or Monitor
Monitor one or more md devices and act on any state changes.
This is only meaningful for RAID1, 4, 5, 6, 10 or multipath
arrays, as only these have interesting state. RAID0 or Linear
never have missing, spare, or failed drives, so there is
nothing to monitor.
Grow Grow (or shrink) an array, or otherwise reshape it in some
way. Currently supported growth options including changing
the active size of component devices and changing the number
of active devices in Linear and RAID levels 0/1/4/5/6,
changing the RAID level between 0, 1, 5, and 6, and between 0
and 10, changing the chunk size and layout for RAID 0,4,5,6,10
as well as adding or removing a write-intent bitmap and
changing the array's consistency policy.
Incremental Assembly
Add a single device to an appropriate array. If the addition
of the device makes the array runnable, the array will be
started. This provides a convenient interface to a hot-plug
system. As each device is detected, mdadm has a chance to
include it in some array as appropriate. Optionally, when the
--fail flag is passed in we will remove the device from any
active array instead of adding it.
If a CONTAINER is passed to mdadm in this mode, then any
arrays within that container will be assembled and started.
Manage This is for doing things to specific components of an array
such as adding new spares and removing faulty devices.
Misc This is an 'everything else' mode that supports operations on
active arrays, operations on component devices such as erasing
old superblocks, and information gathering operations.
Auto-detect
This mode does not act on a specific device or array, but
rather it requests the Linux Kernel to activate any auto-
detected arrays.
-A, --assemble
Assemble a pre-existing array.
-B, --build
Build a legacy array without superblocks.
-C, --create
Create a new array.
-F, --follow, --monitor
Select Monitor mode.
-G, --grow
Change the size or shape of an active array.
-I, --incremental
Add/remove a single device to/from an appropriate array, and
possibly start the array.
--auto-detect
Request that the kernel starts any auto-detected arrays. This
can only work if md is compiled into the kernel — not if it is
a module. Arrays can be auto-detected by the kernel if all
the components are in primary MS-DOS partitions with partition
type FD, and all use v0.90 metadata. In-kernel autodetect is
not recommended for new installations. Using mdadm to detect
and assemble arrays — possibly in an initrd — is substantially
more flexible and should be preferred.
If a device is given before any options, or if the first option is
one of --add, --re-add, --add-spare, --fail, --remove, or --replace,
then the MANAGE mode is assumed. Anything other than these will
cause the Misc mode to be assumed.
-h, --help
Display general help message or, after one of the above
options, a mode-specific help message.
--help-options
Display more detailed help about command line parsing and some
commonly used options.
-V, --version
Print version information for mdadm.
-v, --verbose
Be more verbose about what is happening. This can be used
twice to be extra-verbose. The extra verbosity currently only
affects --detail --scan and --examine --scan.
-q, --quiet
Avoid printing purely informative messages. With this, mdadm
will be silent unless there is something really important to
report.
-f, --force
Be more forceful about certain operations. See the various
modes for the exact meaning of this option in different
contexts.
-c, --config=
Specify the config file or directory. Default is to use
/etc/mdadm.conf and /etc/mdadm.conf.d, or if those are missing
then /etc/mdadm/mdadm.conf and /etc/mdadm/mdadm.conf.d. If
the config file given is partitions then nothing will be read,
but mdadm will act as though the config file contained exactly
DEVICE partitions containers
and will read /proc/partitions to find a list of devices to
scan, and /proc/mdstat to find a list of containers to
examine. If the word none is given for the config file, then
mdadm will act as though the config file were empty.
If the name given is of a directory, then mdadm will collect
all the files contained in the directory with a name ending in
.conf, sort them lexically, and process all of those files as
config files.
-s, --scan
Scan config file or /proc/mdstat for missing information. In
general, this option gives mdadm permission to get any missing
information (like component devices, array devices, array
identities, and alert destination) from the configuration file
(see previous option); one exception is MISC mode when using
--detail or --stop, in which case --scan says to get a list of
array devices from /proc/mdstat.
-e, --metadata=
Declare the style of RAID metadata (superblock) to be used.
The default is 1.2 for --create, and to guess for other
operations. The default can be overridden by setting the
metadata value for the CREATE keyword in mdadm.conf.
Options are:
0, 0.90
Use the original 0.90 format superblock. This format
limits arrays to 28 component devices and limits
component devices of levels 1 and greater to 2
terabytes. It is also possible for there to be
confusion about whether the superblock applies to a
whole device or just the last partition, if that
partition starts on a 64K boundary.
1, 1.0, 1.1, 1.2 default
Use the new version-1 format superblock. This has
fewer restrictions. It can easily be moved between
hosts with different endian-ness, and a recovery
operation can be checkpointed and restarted. The
different sub-versions store the superblock at
different locations on the device, either at the end
(for 1.0), at the start (for 1.1) or 4K from the start
(for 1.2). "1" is equivalent to "1.2" (the commonly
preferred 1.x format). "default" is equivalent to
"1.2".
ddf Use the "Industry Standard" DDF (Disk Data Format)
format defined by SNIA. When creating a DDF array a
CONTAINER will be created, and normal arrays can be
created in that container.
imsm Use the Intel(R) Matrix Storage Manager metadata
format. This creates a CONTAINER which is managed in a
similar manner to DDF, and is supported by an option-
rom on some platforms:
http://www.intel.com/design/chipsets/matrixstorage_sb.htm
--homehost=
This will override any HOMEHOST setting in the config file and
provides the identity of the host which should be considered
the home for any arrays.
When creating an array, the homehost will be recorded in the
metadata. For version-1 superblocks, it will be prefixed to
the array name. For version-0.90 superblocks, part of the
SHA1 hash of the hostname will be stored in the later half of
the UUID.
When reporting information about an array, any array which is
tagged for the given homehost will be reported as such.
When using Auto-Assemble, only arrays tagged for the given
homehost will be allowed to use 'local' names (i.e. not ending
in '_' followed by a digit string). See below under Auto
Assembly.
The special name "any" can be used as a wild card. If an
array is created with --homehost=any then the name "any" will
be stored in the array and it can be assembled in the same way
on any host. If an array is assembled with this option, then
the homehost recorded on the array will be ignored.
--prefer=
When mdadm needs to print the name for a device it normally
finds the name in /dev which refers to the device and is
shortest. When a path component is given with --prefer mdadm
will prefer a longer name if it contains that component. For
example --prefer=by-uuid will prefer a name in a subdirectory
of /dev called by-uuid.
This functionality is currently only provided by --detail and
--monitor.
--home-cluster=
specifies the cluster name for the md device. The md device
can be assembled only on the cluster which matches the name
specified. If this option is not provided, mdadm tries to
detect the cluster name automatically.
-n, --raid-devices=
Specify the number of active devices in the array. This, plus
the number of spare devices (see below) must equal the number
of component-devices (including "missing" devices) that are
listed on the command line for --create. Setting a value of 1
is probably a mistake and so requires that --force be
specified first. A value of 1 will then be allowed for
linear, multipath, RAID0 and RAID1. It is never allowed for
RAID4, RAID5 or RAID6.
This number can only be changed using --grow for RAID1, RAID4,
RAID5 and RAID6 arrays, and only on kernels which provide the
necessary support.
-x, --spare-devices=
Specify the number of spare (eXtra) devices in the initial
array. Spares can also be added and removed later. The
number of component devices listed on the command line must
equal the number of RAID devices plus the number of spare
devices.
-z, --size=
Amount (in Kilobytes) of space to use from each drive in RAID
levels 1/4/5/6. This must be a multiple of the chunk size,
and must leave about 128Kb of space at the end of the drive
for the RAID superblock. If this is not specified (as it
normally is not) the smallest drive (or partition) sets the
size, though if there is a variance among the drives of
greater than 1%, a warning is issued.
A suffix of 'K', 'M' or 'G' can be given to indicate
Kilobytes, Megabytes or Gigabytes respectively.
Sometimes a replacement drive can be a little smaller than the
original drives though this should be minimised by IDEMA
standards. Such a replacement drive will be rejected by md.
To guard against this it can be useful to set the initial size
slightly smaller than the smaller device with the aim that it
will still be larger than any replacement.
This value can be set with --grow for RAID level 1/4/5/6
though CONTAINER based arrays such as those with IMSM metadata
may not be able to support this. If the array was created
with a size smaller than the currently active drives, the
extra space can be accessed using --grow. The size can be
given as max which means to choose the largest size that fits
on all current drives.
Before reducing the size of the array (with --grow --size=)
you should make sure that space isn't needed. If the device
holds a filesystem, you would need to resize the filesystem to
use less space.
After reducing the array size you should check that the data
stored in the device is still available. If the device holds
a filesystem, then an 'fsck' of the filesystem is a minimum
requirement. If there are problems the array can be made
bigger again with no loss with another --grow --size= command.
This value cannot be used when creating a CONTAINER such as
with DDF and IMSM metadata, though it perfectly valid when
creating an array inside a container.
-Z, --array-size=
This is only meaningful with --grow and its effect is not
persistent: when the array is stopped and restarted the
default array size will be restored.
Setting the array-size causes the array to appear smaller to
programs that access the data. This is particularly needed
before reshaping an array so that it will be smaller. As the
reshape is not reversible, but setting the size with --array-
size is, it is required that the array size is reduced as
appropriate before the number of devices in the array is
reduced.
Before reducing the size of the array you should make sure
that space isn't needed. If the device holds a filesystem,
you would need to resize the filesystem to use less space.
After reducing the array size you should check that the data
stored in the device is still available. If the device holds
a filesystem, then an 'fsck' of the filesystem is a minimum
requirement. If there are problems the array can be made
bigger again with no loss with another --grow --array-size=
command.
A suffix of 'K', 'M' or 'G' can be given to indicate
Kilobytes, Megabytes or Gigabytes respectively. A value of
max restores the apparent size of the array to be whatever the
real amount of available space is.
Clustered arrays do not support this parameter yet.
-c, --chunk=
Specify chunk size of kilobytes. The default when creating an
array is 512KB. To ensure compatibility with earlier
versions, the default when building an array with no
persistent metadata is 64KB. This is only meaningful for
RAID0, RAID4, RAID5, RAID6, and RAID10.
RAID4, RAID5, RAID6, and RAID10 require the chunk size to be a
power of 2. In any case it must be a multiple of 4KB.
A suffix of 'K', 'M' or 'G' can be given to indicate
Kilobytes, Megabytes or Gigabytes respectively.
--rounding=
Specify rounding factor for a Linear array. The size of each
component will be rounded down to a multiple of this size.
This is a synonym for --chunk but highlights the different
meaning for Linear as compared to other RAID levels. The
default is 64K if a kernel earlier than 2.6.16 is in use, and
is 0K (i.e. no rounding) in later kernels.
-l, --level=
Set RAID level. When used with --create, options are: linear,
raid0, 0, stripe, raid1, 1, mirror, raid4, 4, raid5, 5, raid6,
6, raid10, 10, multipath, mp, faulty, container. Obviously
some of these are synonymous.
When a CONTAINER metadata type is requested, only the
container level is permitted, and it does not need to be
explicitly given.
When used with --build, only linear, stripe, raid0, 0, raid1,
multipath, mp, and faulty are valid.
Can be used with --grow to change the RAID level in some
cases. See LEVEL CHANGES below.
-p, --layout=
This option configures the fine details of data layout for
RAID5, RAID6, and RAID10 arrays, and controls the failure
modes for faulty.
The layout of the RAID5 parity block can be one of
left-asymmetric, left-symmetric, right-asymmetric,
right-symmetric, la, ra, ls, rs. The default is
left-symmetric.
It is also possible to cause RAID5 to use a RAID4-like layout
by choosing parity-first, or parity-last.
Finally for RAID5 there are DDF-compatible layouts,
ddf-zero-restart, ddf-N-restart, and ddf-N-continue.
These same layouts are available for RAID6. There are also 4
layouts that will provide an intermediate stage for converting
between RAID5 and RAID6. These provide a layout which is
identical to the corresponding RAID5 layout on the first N-1
devices, and has the 'Q' syndrome (the second 'parity' block
used by RAID6) on the last device. These layouts are:
left-symmetric-6, right-symmetric-6, left-asymmetric-6,
right-asymmetric-6, and parity-first-6.
When setting the failure mode for level faulty, the options
are: write-transient, wt, read-transient, rt,
write-persistent, wp, read-persistent, rp, write-all,
read-fixable, rf, clear, flush, none.
Each failure mode can be followed by a number, which is used
as a period between fault generation. Without a number, the
fault is generated once on the first relevant request. With a
number, the fault will be generated after that many requests,
and will continue to be generated every time the period
elapses.
Multiple failure modes can be current simultaneously by using
the --grow option to set subsequent failure modes.
"clear" or "none" will remove any pending or periodic failure
modes, and "flush" will clear any persistent faults.
Finally, the layout options for RAID10 are one of 'n', 'o' or
'f' followed by a small number. The default is 'n2'. The
supported options are:
'n' signals 'near' copies. Multiple copies of one data block
are at similar offsets in different devices.
'o' signals 'offset' copies. Rather than the chunks being
duplicated within a stripe, whole stripes are duplicated but
are rotated by one device so duplicate blocks are on different
devices. Thus subsequent copies of a block are in the next
drive, and are one chunk further down.
'f' signals 'far' copies (multiple copies have very different
offsets). See md(4) for more detail about 'near', 'offset',
and 'far'.
The number is the number of copies of each datablock. 2 is
normal, 3 can be useful. This number can be at most equal to
the number of devices in the array. It does not need to
divide evenly into that number (e.g. it is perfectly legal to
have an 'n2' layout for an array with an odd number of
devices).
When an array is converted between RAID5 and RAID6 an
intermediate RAID6 layout is used in which the second parity
block (Q) is always on the last device. To convert a RAID5 to
RAID6 and leave it in this new layout (which does not require
re-striping) use --layout=preserve. This will try to avoid
any restriping.
The converse of this is --layout=normalise which will change a
non-standard RAID6 layout into a more standard arrangement.
--parity=
same as --layout (thus explaining the p of -p).
-b, --bitmap=
Specify a file to store a write-intent bitmap in. The file
should not exist unless --force is also given. The same file
should be provided when assembling the array. If the word
internal is given, then the bitmap is stored with the metadata
on the array, and so is replicated on all devices. If the
word none is given with --grow mode, then any bitmap that is
present is removed. If the word clustered is given, the array
is created for a clustered environment. One bitmap is created
for each node as defined by the --nodes parameter and are
stored internally.
To help catch typing errors, the filename must contain at
least one slash ('/') if it is a real file (not 'internal' or
'none').
Note: external bitmaps are only known to work on ext2 and
ext3. Storing bitmap files on other filesystems may result in
serious problems.
When creating an array on devices which are 100G or larger,
mdadm automatically adds an internal bitmap as it will usually
be beneficial. This can be suppressed with --bitmap=none or
by selecting a different consistency policy with
--consistency-policy.
--bitmap-chunk=
Set the chunksize of the bitmap. Each bit corresponds to that
many Kilobytes of storage. When using a file based bitmap,
the default is to use the smallest size that is at-least 4 and
requires no more than 2^21 chunks. When using an internal
bitmap, the chunksize defaults to 64Meg, or larger if
necessary to fit the bitmap into the available space.
A suffix of 'K', 'M' or 'G' can be given to indicate
Kilobytes, Megabytes or Gigabytes respectively.
-W, --write-mostly
subsequent devices listed in a --build, --create, or --add
command will be flagged as 'write-mostly'. This is valid for
RAID1 only and means that the 'md' driver will avoid reading
from these devices if at all possible. This can be useful if
mirroring over a slow link.
--write-behind=
Specify that write-behind mode should be enabled (valid for
RAID1 only). If an argument is specified, it will set the
maximum number of outstanding writes allowed. The default
value is 256. A write-intent bitmap is required in order to
use write-behind mode, and write-behind is only attempted on
drives marked as write-mostly.
--failfast
subsequent devices listed in a --create or --add command will
be flagged as 'failfast'. This is valid for RAID1 and RAID10
only. IO requests to these devices will be encouraged to fail
quickly rather than cause long delays due to error handling.
Also no attempt is made to repair a read error on these
devices.
If an array becomes degraded so that the 'failfast' device is
the only usable device, the 'failfast' flag will then be
ignored and extended delays will be preferred to complete
failure.
The 'failfast' flag is appropriate for storage arrays which
have a low probability of true failure, but which may
sometimes cause unacceptable delays due to internal
maintenance functions.
--assume-clean
Tell mdadm that the array pre-existed and is known to be
clean. It can be useful when trying to recover from a major
failure as you can be sure that no data will be affected
unless you actually write to the array. It can also be used
when creating a RAID1 or RAID10 if you want to avoid the
initial resync, however this practice — while normally safe —
is not recommended. Use this only if you really know what you
are doing.
When the devices that will be part of a new array were filled
with zeros before creation the operator knows the array is
actually clean. If that is the case, such as after running
badblocks, this argument can be used to tell mdadm the facts
the operator knows.
When an array is resized to a larger size with --grow --size=
the new space is normally resynced in that same way that the
whole array is resynced at creation. From Linux version 3.0,
--assume-clean can be used with that command to avoid the
automatic resync.
--backup-file=
This is needed when --grow is used to increase the number of
raid-devices in a RAID5 or RAID6 if there are no spare devices
available, or to shrink, change RAID level or layout. See the
GROW MODE section below on RAID-DEVICES CHANGES. The file
must be stored on a separate device, not on the RAID array
being reshaped.
--data-offset=
Arrays with 1.x metadata can leave a gap between the start of
the device and the start of array data. This gap can be used
for various metadata. The start of data is known as the
data-offset. Normally an appropriate data offset is computed
automatically. However it can be useful to set it explicitly
such as when re-creating an array which was originally created
using a different version of mdadm which computed a different
offset.
Setting the offset explicitly over-rides the default. The
value given is in Kilobytes unless a suffix of 'K', 'M' or 'G'
is used to explicitly indicate Kilobytes, Megabytes or
Gigabytes respectively.
Since Linux 3.4, --data-offset can also be used with --grow
for some RAID levels (initially on RAID10). This allows the
data-offset to be changed as part of the reshape process.
When the data offset is changed, no backup file is required as
the difference in offsets is used to provide the same
functionality.
When the new offset is earlier than the old offset, the number
of devices in the array cannot shrink. When it is after the
old offset, the number of devices in the array cannot
increase.
When creating an array, --data-offset can be specified as
variable. In the case each member device is expected to have
a offset appended to the name, separated by a colon. This
makes it possible to recreate exactly an array which has
varying data offsets (as can happen when different versions of
mdadm are used to add different devices).
--continue
This option is complementary to the --freeze-reshape option
for assembly. It is needed when --grow operation is
interrupted and it is not restarted automatically due to
--freeze-reshape usage during array assembly. This option is
used together with -G , ( --grow ) command and device for a
pending reshape to be continued. All parameters required for
reshape continuation will be read from array metadata. If
initial --grow command had required --backup-file= option to
be set, continuation option will require to have exactly the
same backup file given as well.
Any other parameter passed together with --continue option
will be ignored.
-N, --name=
Set a name for the array. This is currently only effective
when creating an array with a version-1 superblock, or an
array in a DDF container. The name is a simple textual string
that can be used to identify array components when assembling.
If name is needed but not specified, it is taken from the
basename of the device that is being created. e.g. when
creating /dev/md/home the name will default to home.
-R, --run
Insist that mdadm run the array, even if some of the
components appear to be active in another array or filesystem.
Normally mdadm will ask for confirmation before including such
components in an array. This option causes that question to
be suppressed.
-f, --force
Insist that mdadm accept the geometry and layout specified
without question. Normally mdadm will not allow creation of
an array with only one device, and will try to create a RAID5
array with one missing drive (as this makes the initial resync
work faster). With --force, mdadm will not try to be so
clever.
-o, --readonly
Start the array read only rather than read-write as normal.
No writes will be allowed to the array, and no resync,
recovery, or reshape will be started. It works with Create,
Assemble, Manage and Misc mode.
-a, --auto{=yes,md,mdp,part,p}{NN}
Instruct mdadm how to create the device file if needed,
possibly allocating an unused minor number. "md" causes a
non-partitionable array to be used (though since Linux 2.6.28,
these array devices are in fact partitionable). "mdp", "part"
or "p" causes a partitionable array (2.6 and later) to be
used. "yes" requires the named md device to have a 'standard'
format, and the type and minor number will be determined from
this. With mdadm 3.0, device creation is normally left up to
udev so this option is unlikely to be needed. See DEVICE
NAMES below.
The argument can also come immediately after "-a". e.g.
"-ap".
If --auto is not given on the command line or in the config
file, then the default will be --auto=yes.
If --scan is also given, then any auto= entries in the config
file will override the --auto instruction given on the command
line.
For partitionable arrays, mdadm will create the device file
for the whole array and for the first 4 partitions. A
different number of partitions can be specified at the end of
this option (e.g. --auto=p7). If the device name ends with a
digit, the partition names add a 'p', and a number, e.g.
/dev/md/home1p3. If there is no trailing digit, then the
partition names just have a number added, e.g.
/dev/md/scratch3.
If the md device name is in a 'standard' format as described
in DEVICE NAMES, then it will be created, if necessary, with
the appropriate device number based on that name. If the
device name is not in one of these formats, then a unused
device number will be allocated. The device number will be
considered unused if there is no active array for that number,
and there is no entry in /dev for that number and with a non-
standard name. Names that are not in 'standard' format are
only allowed in "/dev/md/".
This is meaningful with --create or --build.
-a, --add
This option can be used in Grow mode in two cases.
If the target array is a Linear array, then --add can be used
to add one or more devices to the array. They are simply
catenated on to the end of the array. Once added, the devices
cannot be removed.
If the --raid-disks option is being used to increase the
number of devices in an array, then --add can be used to add
some extra devices to be included in the array. In most cases
this is not needed as the extra devices can be added as spares
first, and then the number of raid-disks can be changed.
However for RAID0, it is not possible to add spares. So to
increase the number of devices in a RAID0, it is necessary to
set the new number of devices, and to add the new devices, in
the same command.
--nodes
Only works when the array is for clustered environment. It
specifies the maximum number of nodes in the cluster that will
use this device simultaneously. If not specified, this
defaults to 4.
--write-journal
Specify journal device for the RAID-4/5/6 array. The journal
device should be a SSD with reasonable lifetime.
--symlinks
Auto creation of symlinks in /dev to /dev/md, option
--symlinks must be 'no' or 'yes' and work with --create and
--build.
-k, --consistency-policy=
Specify how the array maintains consistency in case of
unexpected shutdown. Only relevant for RAID levels with
redundancy. Currently supported options are:
resync Full resync is performed and all redundancy is
regenerated when the array is started after unclean
shutdown.
bitmap Resync assisted by a write-intent bitmap. Implicitly
selected when using --bitmap.
journal
For RAID levels 4/5/6, journal device is used to log
transactions and replay after unclean shutdown.
Implicitly selected when using --write-journal.
ppl For RAID5 only, Partial Parity Log is used to close the
write hole and eliminate resync. PPL is stored in the
metadata region of RAID member drives, no additional
journal drive is needed.
Can be used with --grow to change the consistency policy of an
active array in some cases. See CONSISTENCY POLICY CHANGES
below.
-u, --uuid=
uuid of array to assemble. Devices which don't have this uuid
are excluded
-m, --super-minor=
Minor number of device that array was created for. Devices
which don't have this minor number are excluded. If you
create an array as /dev/md1, then all superblocks will contain
the minor number 1, even if the array is later assembled as
/dev/md2.
Giving the literal word "dev" for --super-minor will cause
mdadm to use the minor number of the md device that is being
assembled. e.g. when assembling /dev/md0, --super-minor=dev
will look for super blocks with a minor number of 0.
--super-minor is only relevant for v0.90 metadata, and should
not normally be used. Using --uuid is much safer.
-N, --name=
Specify the name of the array to assemble. This must be the
name that was specified when creating the array. It must
either match the name stored in the superblock exactly, or it
must match with the current homehost prefixed to the start of
the given name.
-f, --force
Assemble the array even if the metadata on some devices
appears to be out-of-date. If mdadm cannot find enough
working devices to start the array, but can find some devices
that are recorded as having failed, then it will mark those
devices as working so that the array can be started. An array
which requires --force to be started may contain data
corruption. Use it carefully.
-R, --run
Attempt to start the array even if fewer drives were given
than were present last time the array was active. Normally if
not all the expected drives are found and --scan is not used,
then the array will be assembled but not started. With --run
an attempt will be made to start it anyway.
--no-degraded
This is the reverse of --run in that it inhibits the startup
of array unless all expected drives are present. This is only
needed with --scan, and can be used if the physical
connections to devices are not as reliable as you would like.
-a, --auto{=no,yes,md,mdp,part}
See this option under Create and Build options.
-b, --bitmap=
Specify the bitmap file that was given when the array was
created. If an array has an internal bitmap, there is no need
to specify this when assembling the array.
--backup-file=
If --backup-file was used while reshaping an array (e.g.
changing number of devices or chunk size) and the system
crashed during the critical section, then the same
--backup-file must be presented to --assemble to allow
possibly corrupted data to be restored, and the reshape to be
completed.
--invalid-backup
If the file needed for the above option is not available for
any reason an empty file can be given together with this
option to indicate that the backup file is invalid. In this
case the data that was being rearranged at the time of the
crash could be irrecoverably lost, but the rest of the array
may still be recoverable. This option should only be used as
a last resort if there is no way to recover the backup file.
-U, --update=
Update the superblock on each device while assembling the
array. The argument given to this flag can be one of
sparc2.2, summaries, uuid, name, nodes, homehost, home-
cluster, resync, byteorder, devicesize, no-bitmap, bbl,
no-bbl, ppl, no-ppl, metadata, or super-minor.
The sparc2.2 option will adjust the superblock of an array
what was created on a Sparc machine running a patched 2.2
Linux kernel. This kernel got the alignment of part of the
superblock wrong. You can use the --examine --sparc2.2 option
to mdadm to see what effect this would have.
The super-minor option will update the preferred minor field
on each superblock to match the minor number of the array
being assembled. This can be useful if --examine reports a
different "Preferred Minor" to --detail. In some cases this
update will be performed automatically by the kernel driver.
In particular the update happens automatically at the first
write to an array with redundancy (RAID level 1 or greater) on
a 2.6 (or later) kernel.
The uuid option will change the uuid of the array. If a UUID
is given with the --uuid option that UUID will be used as a
new UUID and will NOT be used to help identify the devices in
the array. If no --uuid is given, a random UUID is chosen.
The name option will change the name of the array as stored in
the superblock. This is only supported for version-1
superblocks.
The nodes option will change the nodes of the array as stored
in the bitmap superblock. This option only works for a
clustered environment.
The homehost option will change the homehost as recorded in
the superblock. For version-0 superblocks, this is the same
as updating the UUID. For version-1 superblocks, this
involves updating the name.
The home-cluster option will change the cluster name as
recorded in the superblock and bitmap. This option only works
for clustered environment.
The resync option will cause the array to be marked dirty
meaning that any redundancy in the array (e.g. parity for
RAID5, copies for RAID1) may be incorrect. This will cause
the RAID system to perform a "resync" pass to make sure that
all redundant information is correct.
The byteorder option allows arrays to be moved between
machines with different byte-order, such as from a big-endian
machine like a Sparc or some MIPS machines, to a little-endian
x86_64 machine. When assembling such an array for the first
time after a move, giving --update=byteorder will cause mdadm
to expect superblocks to have their byteorder reversed, and
will correct that order before assembling the array. This is
only valid with original (Version 0.90) superblocks.
The summaries option will correct the summaries in the
superblock. That is the counts of total, working, active,
failed, and spare devices.
The devicesize option will rarely be of use. It applies to
version 1.1 and 1.2 metadata only (where the metadata is at
the start of the device) and is only useful when the component
device has changed size (typically become larger). The
version 1 metadata records the amount of the device that can
be used to store data, so if a device in a version 1.1 or 1.2
array becomes larger, the metadata will still be visible, but
the extra space will not. In this case it might be useful to
assemble the array with --update=devicesize. This will cause
mdadm to determine the maximum usable amount of space on each
device and update the relevant field in the metadata.
The metadata option only works on v0.90 metadata arrays and
will convert them to v1.0 metadata. The array must not be
dirty (i.e. it must not need a sync) and it must not have a
write-intent bitmap.
The old metadata will remain on the devices, but will appear
older than the new metadata and so will usually be ignored.
The old metadata (or indeed the new metadata) can be removed
by giving the appropriate --metadata= option to
--zero-superblock.
The no-bitmap option can be used when an array has an internal
bitmap which is corrupt in some way so that assembling the
array normally fails. It will cause any internal bitmap to be
ignored.
The bbl option will reserve space in each device for a bad
block list. This will be 4K in size and positioned near the
end of any free space between the superblock and the data.
The no-bbl option will cause any reservation of space for a
bad block list to be removed. If the bad block list contains
entries, this will fail, as removing the list could cause data
corruption.
The ppl option will enable PPL for a RAID5 array and reserve
space for PPL on each device. There must be enough free space
between the data and superblock and a write-intent bitmap or
journal must not be used.
The no-ppl option will disable PPL in the superblock.
--freeze-reshape
Option is intended to be used in start-up scripts during
initrd boot phase. When array under reshape is assembled
during initrd phase, this option stops reshape after reshape
critical section is being restored. This happens before file
system pivot operation and avoids loss of file system context.
Losing file system context would cause reshape to be broken.
Reshape can be continued later using the --continue option for
the grow command.
--symlinks
See this option under Create and Build options.
-t, --test
Unless a more serious error occurred, mdadm will exit with a
status of 2 if no changes were made to the array and 0 if at
least one change was made. This can be useful when an
indirect specifier such as missing, detached or faulty is used
in requesting an operation on the array. --test will report
failure if these specifiers didn't find any match.
-a, --add
hot-add listed devices. If a device appears to have recently
been part of the array (possibly it failed or was removed) the
device is re-added as described in the next point. If that
fails or the device was never part of the array, the device is
added as a hot-spare. If the array is degraded, it will
immediately start to rebuild data onto that spare.
Note that this and the following options are only meaningful
on array with redundancy. They don't apply to RAID0 or
Linear.
--re-add
re-add a device that was previously removed from an array. If
the metadata on the device reports that it is a member of the
array, and the slot that it used is still vacant, then the
device will be added back to the array in the same position.
This will normally cause the data for that device to be
recovered. However based on the event count on the device,
the recovery may only require sections that are flagged a
write-intent bitmap to be recovered or may not require any
recovery at all.
When used on an array that has no metadata (i.e. it was built
with --build) it will be assumed that bitmap-based recovery is
enough to make the device fully consistent with the array.
When used with v1.x metadata, --re-add can be accompanied by
--update=devicesize, --update=bbl, or --update=no-bbl. See
the description of these option when used in Assemble mode for
an explanation of their use.
If the device name given is missing then mdadm will try to
find any device that looks like it should be part of the array
but isn't and will try to re-add all such devices.
If the device name given is faulty then mdadm will find all
devices in the array that are marked faulty, remove them and
attempt to immediately re-add them. This can be useful if you
are certain that the reason for failure has been resolved.
--add-spare
Add a device as a spare. This is similar to --add except that
it does not attempt --re-add first. The device will be added
as a spare even if it looks like it could be an recent member
of the array.
-r, --remove
remove listed devices. They must not be active. i.e. they
should be failed or spare devices.
As well as the name of a device file (e.g. /dev/sda1) the
words failed, detached and names like set-A can be given to
--remove. The first causes all failed device to be removed.
The second causes any device which is no longer connected to
the system (i.e an 'open' returns ENXIO) to be removed. The
third will remove a set as describe below under --fail.
-f, --fail
Mark listed devices as faulty. As well as the name of a
device file, the word detached or a set name like set-A can be
given. The former will cause any device that has been
detached from the system to be marked as failed. It can then
be removed.
For RAID10 arrays where the number of copies evenly divides
the number of devices, the devices can be conceptually divided
into sets where each set contains a single complete copy of
the data on the array. Sometimes a RAID10 array will be
configured so that these sets are on separate controllers. In
this case all the devices in one set can be failed by giving a
name like set-A or set-B to --fail. The appropriate set names
are reported by --detail.
--set-faulty
same as --fail.
--replace
Mark listed devices as requiring replacement. As soon as a
spare is available, it will be rebuilt and will replace the
marked device. This is similar to marking a device as faulty,
but the device remains in service during the recovery process
to increase resilience against multiple failures. When the
replacement process finishes, the replaced device will be
marked as faulty.
--with This can follow a list of --replace devices. The devices
listed after --with will be preferentially used to replace the
devices listed after --replace. These device must already be
spare devices in the array.
--write-mostly
Subsequent devices that are added or re-added will have the
'write-mostly' flag set. This is only valid for RAID1 and
means that the 'md' driver will avoid reading from these
devices if possible.
--readwrite
Subsequent devices that are added or re-added will have the
'write-mostly' flag cleared.
--cluster-confirm
Confirm the existence of the device. This is issued in
response to an --add request by a node in a cluster. When a
node adds a device it sends a message to all nodes in the
cluster to look for a device with a UUID. This translates to a
udev notification with the UUID of the device to be added and
the slot number. The receiving node must acknowledge this
message with --cluster-confirm. Valid arguments are
<slot>:<devicename> in case the device is found or
<slot>:missing in case the device is not found.
--add-journal
Add journal to an existing array, or recreate journal for
RAID-4/5/6 array that lost a journal device. To avoid
interrupting on-going write opertions, --add-journal only
works for array in Read-Only state.
--failfast
Subsequent devices that are added or re-added will have the
'failfast' flag set. This is only valid for RAID1 and RAID10
and means that the 'md' driver will avoid long timeouts on
error handling where possible.
--nofailfast
Subsequent devices that are re-added will be re-added without
the 'failfast' flag set.
Each of these options requires that the first device listed is the
array to be acted upon, and the remainder are component devices to be
added, removed, marked as faulty, etc. Several different operations
can be specified for different devices, e.g.
mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove
/dev/sdb1
Each operation applies to all devices listed until the next
operation.
If an array is using a write-intent bitmap, then devices which have
been removed can be re-added in a way that avoids a full
reconstruction but instead just updates the blocks that have changed
since the device was removed. For arrays with persistent metadata
(superblocks) this is done automatically. For arrays created with
--build mdadm needs to be told that this device we removed recently
with --re-add.
Devices can only be removed from an array if they are not in active
use, i.e. that must be spares or failed devices. To remove an active
device, it must first be marked as faulty.
-Q, --query
Examine a device to see (1) if it is an md device and (2) if
it is a component of an md array. Information about what is
discovered is presented.
-D, --detail
Print details of one or more md devices.
--detail-platform
Print details of the platform's RAID capabilities (firmware /
hardware topology) for a given metadata format. If used
without argument, mdadm will scan all controllers looking for
their capabilities. Otherwise, mdadm will only look at the
controller specified by the argument in form of an absolute
filepath or a link, e.g.
/sys/devices/pci0000:00/0000:00:1f.2.
-Y, --export
When used with --detail, --detail-platform, --examine, or
--incremental output will be formatted as key=value pairs for
easy import into the environment.
With --incremental The value MD_STARTED indicates whether an
array was started (yes) or not, which may include a reason
(unsafe, nothing, no). Also the value MD_FOREIGN indicates if
the array is expected on this host (no), or seems to be from
elsewhere (yes).
-E, --examine
Print contents of the metadata stored on the named device(s).
Note the contrast between --examine and --detail. --examine
applies to devices which are components of an array, while
--detail applies to a whole array which is currently active.
--sparc2.2
If an array was created on a SPARC machine with a 2.2 Linux
kernel patched with RAID support, the superblock will have
been created incorrectly, or at least incompatibly with 2.4
and later kernels. Using the --sparc2.2 flag with --examine
will fix the superblock before displaying it. If this appears
to do the right thing, then the array can be successfully
assembled using --assemble --update=sparc2.2.
-X, --examine-bitmap
Report information about a bitmap file. The argument is
either an external bitmap file or an array component in case
of an internal bitmap. Note that running this on an array
device (e.g. /dev/md0) does not report the bitmap for that
array.
--examine-badblocks
List the bad-blocks recorded for the device, if a bad-blocks
list has been configured. Currently only 1.x metadata
supports bad-blocks lists.
--dump=directory
--restore=directory
Save metadata from lists devices, or restore metadata to
listed devices.
-R, --run
start a partially assembled array. If --assemble did not find
enough devices to fully start the array, it might leaving it
partially assembled. If you wish, you can then use --run to
start the array in degraded mode.
-S, --stop
deactivate array, releasing all resources.
-o, --readonly
mark array as readonly.
-w, --readwrite
mark array as readwrite.
--zero-superblock
If the device contains a valid md superblock, the block is
overwritten with zeros. With --force the block where the
superblock would be is overwritten even if it doesn't appear
to be valid.
--kill-subarray=
If the device is a container and the argument to
--kill-subarray specifies an inactive subarray in the
container, then the subarray is deleted. Deleting all
subarrays will leave an 'empty-container' or spare superblock
on the drives. See --zero-superblock for completely removing
a superblock. Note that some formats depend on the subarray
index for generating a UUID, this command will fail if it
would change the UUID of an active subarray.
--update-subarray=
If the device is a container and the argument to
--update-subarray specifies a subarray in the container, then
attempt to update the given superblock field in the subarray.
See below in MISC MODE for details.
-t, --test
When used with --detail, the exit status of mdadm is set to
reflect the status of the device. See below in MISC MODE for
details.
-W, --wait
For each md device given, wait for any resync, recovery, or
reshape activity to finish before returning. mdadm will
return with success if it actually waited for every device
listed, otherwise it will return failure.
--wait-clean
For each md device given, or each device in /proc/mdstat if
--scan is given, arrange for the array to be marked clean as
soon as possible. mdadm will return with success if the array
uses external metadata and we successfully waited. For native
arrays this returns immediately as the kernel handles dirty-
clean transitions at shutdown. No action is taken if safe-
mode handling is disabled.
--action=
Set the "sync_action" for all md devices given to one of idle,
frozen, check, repair. Setting to idle will abort any
currently running action though some actions will
automatically restart. Setting to frozen will abort any
current action and ensure no other action starts
automatically.
Details of check and repair can be found it md(4) under
SCRUBBING AND MISMATCHES.
--rebuild-map, -r
Rebuild the map file (/run/mdadm/map) that mdadm uses to help
track which arrays are currently being assembled.
--run, -R
Run any array assembled as soon as a minimal number of devices
are available, rather than waiting until all expected devices
are present.
--scan, -s
Only meaningful with -R this will scan the map file for arrays
that are being incrementally assembled and will try to start
any that are not already started. If any such array is listed
in mdadm.conf as requiring an external bitmap, that bitmap
will be attached first.
--fail, -f
This allows the hot-plug system to remove devices that have
fully disappeared from the kernel. It will first fail and
then remove the device from any array it belongs to. The
device name given should be a kernel device name such as
"sda", not a name in /dev.
--path=
Only used with --fail. The 'path' given will be recorded so
that if a new device appears at the same location it can be
automatically added to the same array. This allows the failed
device to be automatically replaced by a new device without
metadata if it appears at specified path. This option is
normally only set by a udev script.
-m, --mail
Give a mail address to send alerts to.
-p, --program, --alert
Give a program to be run whenever an event is detected.
-y, --syslog
Cause all events to be reported through 'syslog'. The
messages have facility of 'daemon' and varying priorities.
-d, --delay
Give a delay in seconds. mdadm polls the md arrays and then
waits this many seconds before polling again. The default is
60 seconds. Since 2.6.16, there is no need to reduce this as
the kernel alerts mdadm immediately when there is any change.
-r, --increment
Give a percentage increment. mdadm will generate RebuildNN
events with the given percentage increment.
-f, --daemonise
Tell mdadm to run as a background daemon if it decides to
monitor anything. This causes it to fork and run in the
child, and to disconnect from the terminal. The process id of
the child is written to stdout. This is useful with --scan
which will only continue monitoring if a mail address or alert
program is found in the config file.
-i, --pid-file
When mdadm is running in daemon mode, write the pid of the
daemon process to the specified file, instead of printing it
on standard output.
-1, --oneshot
Check arrays only once. This will generate NewArray events
and more significantly DegradedArray and SparesMissing events.
Running
mdadm --monitor --scan -1
from a cron script will ensure regular notification of any
degraded arrays.
-t, --test
Generate a TestMessage alert for every array found at startup.
This alert gets mailed and passed to the alert program. This
can be used for testing that alert message do get through
successfully.
--no-sharing
This inhibits the functionality for moving spares between
arrays. Only one monitoring process started with --scan but
without this flag is allowed, otherwise the two could
interfere with each other.
Usage: mdadm --assemble md-device options-and-component-devices...
Usage: mdadm --assemble --scan md-devices-and-options...
Usage: mdadm --assemble --scan options...
This usage assembles one or more RAID arrays from pre-existing
components. For each array, mdadm needs to know the md device, the
identity of the array, and a number of component-devices. These can
be found in a number of ways.
In the first usage example (without the --scan) the first device
given is the md device. In the second usage example, all devices
listed are treated as md devices and assembly is attempted. In the
third (where no devices are listed) all md devices that are listed in
the configuration file are assembled. If no arrays are described by
the configuration file, then any arrays that can be found on unused
devices will be assembled.
If precisely one device is listed, but --scan is not given, then
mdadm acts as though --scan was given and identity information is
extracted from the configuration file.
The identity can be given with the --uuid option, the --name option,
or the --super-minor option, will be taken from the md-device record
in the config file, or will be taken from the super block of the
first component-device listed on the command line.
Devices can be given on the --assemble command line or in the config
file. Only devices which have an md superblock which contains the
right identity will be considered for any array.
The config file is only used if explicitly named with --config or
requested with (a possibly implicit) --scan. In the later case,
/etc/mdadm.conf or /etc/mdadm/mdadm.conf is used.
If --scan is not given, then the config file will only be used to
find the identity of md arrays.
Normally the array will be started after it is assembled. However if
--scan is not given and not all expected drives were listed, then the
array is not started (to guard against usage errors). To insist that
the array be started in this case (as may work for RAID1, 4, 5, 6, or
10), give the --run flag.
If udev is active, mdadm does not create any entries in /dev but
leaves that to udev. It does record information in /run/mdadm/map
which will allow udev to choose the correct name.
If mdadm detects that udev is not configured, it will create the
devices in /dev itself.
In Linux kernels prior to version 2.6.28 there were two distinctly
different types of md devices that could be created: one that could
be partitioned using standard partitioning tools and one that could
not. Since 2.6.28 that distinction is no longer relevant as both
type of devices can be partitioned. mdadm will normally create the
type that originally could not be partitioned as it has a well
defined major number (9).
Prior to 2.6.28, it is important that mdadm chooses the correct type
of array device to use. This can be controlled with the --auto
option. In particular, a value of "mdp" or "part" or "p" tells mdadm
to use a partitionable device rather than the default.
In the no-udev case, the value given to --auto can be suffixed by a
number. This tells mdadm to create that number of partition devices
rather than the default of 4.
The value given to --auto can also be given in the configuration file
as a word starting auto= on the ARRAY line for the relevant array.
Auto Assembly
When --assemble is used with --scan and no devices are listed, mdadm
will first attempt to assemble all the arrays listed in the config
file.
If no arrays are listed in the config (other than those marked
<ignore>) it will look through the available devices for possible
arrays and will try to assemble anything that it finds. Arrays which
are tagged as belonging to the given homehost will be assembled and
started normally. Arrays which do not obviously belong to this host
are given names that are expected not to conflict with anything
local, and are started "read-auto" so that nothing is written to any
device until the array is written to. i.e. automatic resync etc is
delayed.
If mdadm finds a consistent set of devices that look like they should
comprise an array, and if the superblock is tagged as belonging to
the given home host, it will automatically choose a device name and
try to assemble the array. If the array uses version-0.90 metadata,
then the minor number as recorded in the superblock is used to create
a name in /dev/md/ so for example /dev/md/3. If the array uses
version-1 metadata, then the name from the superblock is used to
similarly create a name in /dev/md/ (the name will have any 'host'
prefix stripped first).
This behaviour can be modified by the AUTO line in the mdadm.conf
configuration file. This line can indicate that specific metadata
type should, or should not, be automatically assembled. If an array
is found which is not listed in mdadm.conf and has a metadata format
that is denied by the AUTO line, then it will not be assembled. The
AUTO line can also request that all arrays identified as being for
this homehost should be assembled regardless of their metadata type.
See mdadm.conf(5) for further details.
Note: Auto assembly cannot be used for assembling and activating some
arrays which are undergoing reshape. In particular as the
backup-file cannot be given, any reshape which requires a backup-file
to continue cannot be started by auto assembly. An array which is
growing to more devices and has passed the critical section can be
assembled using auto-assembly.
Usage: mdadm --build md-device --chunk=X --level=Y --raid-devices=Z
devices
This usage is similar to --create. The difference is that it creates
an array without a superblock. With these arrays there is no
difference between initially creating the array and subsequently
assembling the array, except that hopefully there is useful data
there in the second case.
The level may raid0, linear, raid1, raid10, multipath, or faulty, or
one of their synonyms. All devices must be listed and the array will
be started once complete. It will often be appropriate to use
--assume-clean with levels raid1 or raid10.
Usage: mdadm --create md-device --chunk=X --level=Y
--raid-devices=Z devices
This usage will initialise a new md array, associate some devices
with it, and activate the array.
The named device will normally not exist when mdadm --create is run,
but will be created by udev once the array becomes active.
As devices are added, they are checked to see if they contain RAID
superblocks or filesystems. They are also checked to see if the
variance in device size exceeds 1%.
If any discrepancy is found, the array will not automatically be run,
though the presence of a --run can override this caution.
To create a "degraded" array in which some devices are missing,
simply give the word "missing" in place of a device name. This will
cause mdadm to leave the corresponding slot in the array empty. For
a RAID4 or RAID5 array at most one slot can be "missing"; for a RAID6
array at most two slots. For a RAID1 array, only one real device
needs to be given. All of the others can be "missing".
When creating a RAID5 array, mdadm will automatically create a
degraded array with an extra spare drive. This is because building
the spare into a degraded array is in general faster than resyncing
the parity on a non-degraded, but not clean, array. This feature can
be overridden with the --force option.
When creating an array with version-1 metadata a name for the array
is required. If this is not given with the --name option, mdadm will
choose a name based on the last component of the name of the device
being created. So if /dev/md3 is being created, then the name 3 will
be chosen. If /dev/md/home is being created, then the name home will
be used.
When creating a partition based array, using mdadm with version-1.x
metadata, the partition type should be set to 0xDA (non fs-data).
This type selection allows for greater precision since using any
other [RAID auto-detect (0xFD) or a GNU/Linux partition (0x83)],
might create problems in the event of array recovery through a live
cdrom.
A new array will normally get a randomly assigned 128bit UUID which
is very likely to be unique. If you have a specific need, you can
choose a UUID for the array by giving the --uuid= option. Be warned
that creating two arrays with the same UUID is a recipe for disaster.
Also, using --uuid= when creating a v0.90 array will silently
override any --homehost= setting.
If the array type supports a write-intent bitmap, and if the devices
in the array exceed 100G is size, an internal write-intent bitmap
will automatically be added unless some other option is explicitly
requested with the --bitmap option or a different consistency policy
is selected with the --consistency-policy option. In any case space
for a bitmap will be reserved so that one can be added later with
--grow --bitmap=internal.
If the metadata type supports it (currently only 1.x and IMSM
metadata), space will be allocated to store a bad block list. This
allows a modest number of bad blocks to be recorded, allowing the
drive to remain in service while only partially functional.
When creating an array within a CONTAINER mdadm can be given either
the list of devices to use, or simply the name of the container. The
former case gives control over which devices in the container will be
used for the array. The latter case allows mdadm to automatically
choose which devices to use based on how much spare space is
available.
The General Management options that are valid with --create are:
--run insist on running the array even if some devices look like
they might be in use.
--readonly
start the array in readonly mode.
Usage: mdadm device options... devices...
This usage will allow individual devices in an array to be failed,
removed or added. It is possible to perform multiple operations with
on command. For example:
mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
will firstly mark /dev/hda1 as faulty in /dev/md0 and will then
remove it from the array and finally add it back in as a spare.
However only one md array can be affected by a single command.
When a device is added to an active array, mdadm checks to see if it
has metadata on it which suggests that it was recently a member of
the array. If it does, it tries to "re-add" the device. If there
have been no changes since the device was removed, or if the array
has a write-intent bitmap which has recorded whatever changes there
were, then the device will immediately become a full member of the
array and those differences recorded in the bitmap will be resolved.
Usage: mdadm options ... devices ...
MISC mode includes a number of distinct operations that operate on
distinct devices. The operations are:
--query
The device is examined to see if it is (1) an active md array,
or (2) a component of an md array. The information discovered
is reported.
--detail
The device should be an active md device. mdadm will display
a detailed description of the array. --brief or --scan will
cause the output to be less detailed and the format to be
suitable for inclusion in mdadm.conf. The exit status of
mdadm will normally be 0 unless mdadm failed to get useful
information about the device(s); however, if the --test option
is given, then the exit status will be:
0 The array is functioning normally.
1 The array has at least one failed device.
2 The array has multiple failed devices such that it is
unusable.
4 There was an error while trying to get information
about the device.
--detail-platform
Print detail of the platform's RAID capabilities (firmware /
hardware topology). If the metadata is specified with -e or
--metadata= then the return status will be:
0 metadata successfully enumerated its platform
components on this system
1 metadata is platform independent
2 metadata failed to find its platform components on this
system
--update-subarray=
If the device is a container and the argument to
--update-subarray specifies a subarray in the container, then
attempt to update the given superblock field in the subarray.
Similar to updating an array in "assemble" mode, the field to
update is selected by -U or --update= option. The supported
options are name, ppl and no-ppl.
The name option updates the subarray name in the metadata, it
may not affect the device node name or the device node symlink
until the subarray is re-assembled. If updating name would
change the UUID of an active subarray this operation is
blocked, and the command will end in an error.
The ppl and no-ppl options enable and disable PPL in the
metadata. Currently supported only for IMSM subarrays.
--examine
The device should be a component of an md array. mdadm will
read the md superblock of the device and display the contents.
If --brief or --scan is given, then multiple devices that are
components of the one array are grouped together and reported
in a single entry suitable for inclusion in mdadm.conf.
Having --scan without listing any devices will cause all
devices listed in the config file to be examined.
--dump=directory
If the device contains RAID metadata, a file will be created
in the directory and the metadata will be written to it. The
file will be the same size as the device and have the metadata
written in the file at the same locate that it exists in the
device. However the file will be "sparse" so that only those
blocks containing metadata will be allocated. The total space
used will be small.
The file name used in the directory will be the base name of
the device. Further if any links appear in /dev/disk/by-id
which point to the device, then hard links to the file will be
created in directory based on these by-id names.
Multiple devices can be listed and their metadata will all be
stored in the one directory.
--restore=directory
This is the reverse of --dump. mdadm will locate a file in
the directory that has a name appropriate for the given device
and will restore metadata from it. Names that match
/dev/disk/by-id names are preferred, however if two of those
refer to different files, mdadm will not choose between them
but will abort the operation.
If a file name is given instead of a directory then mdadm will
restore from that file to a single device, always provided the
size of the file matches that of the device, and the file
contains valid metadata.
--stop The devices should be active md arrays which will be
deactivated, as long as they are not currently in use.
--run This will fully activate a partially assembled md array.
--readonly
This will mark an active array as read-only, providing that it
is not currently being used.
--readwrite
This will change a readonly array back to being read/write.
--scan For all operations except --examine, --scan will cause the
operation to be applied to all arrays listed in /proc/mdstat.
For --examine, --scan causes all devices listed in the config
file to be examined.
-b, --brief
Be less verbose. This is used with --detail and --examine.
Using --brief with --verbose gives an intermediate level of
verbosity.
Usage: mdadm --monitor options... devices...
This usage causes mdadm to periodically poll a number of md arrays
and to report on any events noticed. mdadm will never exit once it
decides that there are arrays to be checked, so it should normally be
run in the background.
As well as reporting events, mdadm may move a spare drive from one
array to another if they are in the same spare-group or domain and if
the destination array has a failed drive but no spares.
If any devices are listed on the command line, mdadm will only
monitor those devices. Otherwise all arrays listed in the
configuration file will be monitored. Further, if --scan is given,
then any other md devices that appear in /proc/mdstat will also be
monitored.
The result of monitoring the arrays is the generation of events.
These events are passed to a separate program (if specified) and may
be mailed to a given E-mail address.
When passing events to a program, the program is run once for each
event, and is given 2 or 3 command-line arguments: the first is the
name of the event (see below), the second is the name of the md
device which is affected, and the third is the name of a related
device if relevant (such as a component device that has failed).
If --scan is given, then a program or an E-mail address must be
specified on the command line or in the config file. If neither are
available, then mdadm will not monitor anything. Without --scan,
mdadm will continue monitoring as long as something was found to
monitor. If no program or email is given, then each event is
reported to stdout.
The different events are:
DeviceDisappeared
An md array which previously was configured appears to no
longer be configured. (syslog priority: Critical)
If mdadm was told to monitor an array which is RAID0 or
Linear, then it will report DeviceDisappeared with the
extra information Wrong-Level. This is because RAID0 and
Linear do not support the device-failed, hot-spare and
resync operations which are monitored.
RebuildStarted
An md array started reconstruction (e.g. recovery, resync,
reshape, check, repair). (syslog priority: Warning)
RebuildNN
Where NN is a two-digit number (ie. 05, 48). This
indicates that rebuild has passed that many percent of the
total. The events are generated with fixed increment since
0. Increment size may be specified with a commandline
option (default is 20). (syslog priority: Warning)
RebuildFinished
An md array that was rebuilding, isn't any more, either
because it finished normally or was aborted. (syslog
priority: Warning)
Fail An active component device of an array has been marked as
faulty. (syslog priority: Critical)
FailSpare
A spare component device which was being rebuilt to
replace a faulty device has failed. (syslog priority:
Critical)
SpareActive
A spare component device which was being rebuilt to
replace a faulty device has been successfully rebuilt and
has been made active. (syslog priority: Info)
NewArray
A new md array has been detected in the /proc/mdstat file.
(syslog priority: Info)
DegradedArray
A newly noticed array appears to be degraded. This
message is not generated when mdadm notices a drive
failure which causes degradation, but only when mdadm
notices that an array is degraded when it first sees the
array. (syslog priority: Critical)
MoveSpare
A spare drive has been moved from one array in a spare-
group or domain to another to allow a failed drive to be
replaced. (syslog priority: Info)
SparesMissing
If mdadm has been told, via the config file, that an array
should have a certain number of spare devices, and mdadm
detects that it has fewer than this number when it first
sees the array, it will report a SparesMissing message.
(syslog priority: Warning)
TestMessage
An array was found at startup, and the --test flag was
given. (syslog priority: Info)
Only Fail, FailSpare, DegradedArray, SparesMissing and TestMessage
cause Email to be sent. All events cause the program to be run. The
program is run with two or three arguments: the event name, the array
device and possibly a second device.
Each event has an associated array device (e.g. /dev/md1) and
possibly a second device. For Fail, FailSpare, and SpareActive the
second device is the relevant component device. For MoveSpare the
second device is the array that the spare was moved from.
For mdadm to move spares from one array to another, the different
arrays need to be labeled with the same spare-group or the spares
must be allowed to migrate through matching POLICY domains in the
configuration file. The spare-group name can be any string; it is
only necessary that different spare groups use different names.
When mdadm detects that an array in a spare group has fewer active
devices than necessary for the complete array, and has no spare
devices, it will look for another array in the same spare group that
has a full complement of working drive and a spare. It will then
attempt to remove the spare from the second drive and add it to the
first. If the removal succeeds but the adding fails, then it is
added back to the original array.
If the spare group for a degraded array is not defined, mdadm will
look at the rules of spare migration specified by POLICY lines in
mdadm.conf and then follow similar steps as above if a matching spare
is found.
The GROW mode is used for changing the size or shape of an active
array. For this to work, the kernel must support the necessary
change. Various types of growth are being added during 2.6
development.
Currently the supported changes include
· change the "size" attribute for RAID1, RAID4, RAID5 and RAID6.
· increase or decrease the "raid-devices" attribute of RAID0,
RAID1, RAID4, RAID5, and RAID6.
· change the chunk-size and layout of RAID0, RAID4, RAID5, RAID6
and RAID10.
· convert between RAID1 and RAID5, between RAID5 and RAID6, between
RAID0, RAID4, and RAID5, and between RAID0 and RAID10 (in the
near-2 mode).
· add a write-intent bitmap to any array which supports these
bitmaps, or remove a write-intent bitmap from such an array.
· change the array's consistency policy.
Using GROW on containers is currently supported only for Intel's IMSM
container format. The number of devices in a container can be
increased - which affects all arrays in the container - or an array
in a container can be converted between levels where those levels are
supported by the container, and the conversion is on of those listed
above. Resizing arrays in an IMSM container with --grow --size is
not yet supported.
Grow functionality (e.g. expand a number of raid devices) for Intel's
IMSM container format has an experimental status. It is guarded by
the MDADM_EXPERIMENTAL environment variable which must be set to '1'
for a GROW command to succeed. This is for the following reasons:
1. Intel's native IMSM check-pointing is not fully tested yet.
This can causes IMSM incompatibility during the grow process:
an array which is growing cannot roam between Microsoft
Windows(R) and Linux systems.
2. Interrupting a grow operation is not recommended, because it
has not been fully tested for Intel's IMSM container format
yet.
Note: Intel's native checkpointing doesn't use --backup-file option
and it is transparent for assembly feature.
SIZE CHANGES
Normally when an array is built the "size" is taken from the smallest
of the drives. If all the small drives in an arrays are, one at a
time, removed and replaced with larger drives, then you could have an
array of large drives with only a small amount used. In this
situation, changing the "size" with "GROW" mode will allow the extra
space to start being used. If the size is increased in this way, a
"resync" process will start to make sure the new parts of the array
are synchronised.
Note that when an array changes size, any filesystem that may be
stored in the array will not automatically grow or shrink to use or
vacate the space. The filesystem will need to be explicitly told to
use the extra space after growing, or to reduce its size prior to
shrinking the array.
Also the size of an array cannot be changed while it has an active
bitmap. If an array has a bitmap, it must be removed before the size
can be changed. Once the change is complete a new bitmap can be
created.
Note: --grow --size is not yet supported for external file bitmap.
RAID-DEVICES CHANGES
A RAID1 array can work with any number of devices from 1 upwards
(though 1 is not very useful). There may be times which you want to
increase or decrease the number of active devices. Note that this is
different to hot-add or hot-remove which changes the number of
inactive devices.
When reducing the number of devices in a RAID1 array, the slots which
are to be removed from the array must already be vacant. That is,
the devices which were in those slots must be failed and removed.
When the number of devices is increased, any hot spares that are
present will be activated immediately.
Changing the number of active devices in a RAID5 or RAID6 is much
more effort. Every block in the array will need to be read and
written back to a new location. From 2.6.17, the Linux Kernel is
able to increase the number of devices in a RAID5 safely, including
restarting an interrupted "reshape". From 2.6.31, the Linux Kernel
is able to increase or decrease the number of devices in a RAID5 or
RAID6.
From 2.6.35, the Linux Kernel is able to convert a RAID0 in to a
RAID4 or RAID5. mdadm uses this functionality and the ability to add
devices to a RAID4 to allow devices to be added to a RAID0. When
requested to do this, mdadm will convert the RAID0 to a RAID4, add
the necessary disks and make the reshape happen, and then convert the
RAID4 back to RAID0.
When decreasing the number of devices, the size of the array will
also decrease. If there was data in the array, it could get
destroyed and this is not reversible, so you should firstly shrink
the filesystem on the array to fit within the new size. To help
prevent accidents, mdadm requires that the size of the array be
decreased first with mdadm --grow --array-size. This is a reversible
change which simply makes the end of the array inaccessible. The
integrity of any data can then be checked before the non-reversible
reduction in the number of devices is request.
When relocating the first few stripes on a RAID5 or RAID6, it is not
possible to keep the data on disk completely consistent and crash-
proof. To provide the required safety, mdadm disables writes to the
array while this "critical section" is reshaped, and takes a backup
of the data that is in that section. For grows, this backup may be
stored in any spare devices that the array has, however it can also
be stored in a separate file specified with the --backup-file option,
and is required to be specified for shrinks, RAID level changes and
layout changes. If this option is used, and the system does crash
during the critical period, the same file must be passed to
--assemble to restore the backup and reassemble the array. When
shrinking rather than growing the array, the reshape is done from the
end towards the beginning, so the "critical section" is at the end of
the reshape.
LEVEL CHANGES
Changing the RAID level of any array happens instantaneously.
However in the RAID5 to RAID6 case this requires a non-standard
layout of the RAID6 data, and in the RAID6 to RAID5 case that non-
standard layout is required before the change can be accomplished.
So while the level change is instant, the accompanying layout change
can take quite a long time. A --backup-file is required. If the
array is not simultaneously being grown or shrunk, so that the array
size will remain the same - for example, reshaping a 3-drive RAID5
into a 4-drive RAID6 - the backup file will be used not just for a
"cricital section" but throughout the reshape operation, as described
below under LAYOUT CHANGES.
CHUNK-SIZE AND LAYOUT CHANGES
Changing the chunk-size of layout without also changing the number of
devices as the same time will involve re-writing all blocks in-place.
To ensure against data loss in the case of a crash, a --backup-file
must be provided for these changes. Small sections of the array will
be copied to the backup file while they are being rearranged. This
means that all the data is copied twice, once to the backup and once
to the new layout on the array, so this type of reshape will go very
slowly.
If the reshape is interrupted for any reason, this backup file must
be made available to mdadm --assemble so the array can be
reassembled. Consequently the file cannot be stored on the device
being reshaped.
BITMAP CHANGES
A write-intent bitmap can be added to, or removed from, an active
array. Either internal bitmaps, or bitmaps stored in a separate
file, can be added. Note that if you add a bitmap stored in a file
which is in a filesystem that is on the RAID array being affected,
the system will deadlock. The bitmap must be on a separate
filesystem.
CONSISTENCY POLICY CHANGES
The consistency policy of an active array can be changed by using the
--consistency-policy option in Grow mode. Currently this works only
for the ppl and resync policies and allows to enable or disable the
RAID5 Partial Parity Log (PPL).
Usage: mdadm --incremental [--run] [--quiet] component-device
[optional-aliases-for-device]
Usage: mdadm --incremental --fail component-device
Usage: mdadm --incremental --rebuild-map
Usage: mdadm --incremental --run --scan
This mode is designed to be used in conjunction with a device
discovery system. As devices are found in a system, they can be
passed to mdadm --incremental to be conditionally added to an
appropriate array.
Conversely, it can also be used with the --fail flag to do just the
opposite and find whatever array a particular device is part of and
remove the device from that array.
If the device passed is a CONTAINER device created by a previous call
to mdadm, then rather than trying to add that device to an array, all
the arrays described by the metadata of the container will be
started.
mdadm performs a number of tests to determine if the device is part
of an array, and which array it should be part of. If an appropriate
array is found, or can be created, mdadm adds the device to the array
and conditionally starts the array.
Note that mdadm will normally only add devices to an array which were
previously working (active or spare) parts of that array. The
support for automatic inclusion of a new drive as a spare in some
array requires a configuration through POLICY in config file.
The tests that mdadm makes are as follow:
+ Is the device permitted by mdadm.conf? That is, is it listed
in a DEVICES line in that file. If DEVICES is absent then the
default it to allow any device. Similarly if DEVICES contains
the special word partitions then any device is allowed.
Otherwise the device name given to mdadm, or one of the
aliases given, or an alias found in the filesystem, must match
one of the names or patterns in a DEVICES line.
This is the only context where the aliases are used. They are
usually provided by a udev rules mentioning $env{DEVLINKS}.
+ Does the device have a valid md superblock? If a specific
metadata version is requested with --metadata or -e then only
that style of metadata is accepted, otherwise mdadm finds any
known version of metadata. If no md metadata is found, the
device may be still added to an array as a spare if POLICY
allows.
mdadm keeps a list of arrays that it has partially assembled in
/run/mdadm/map. If no array exists which matches the metadata on the
new device, mdadm must choose a device name and unit number. It does
this based on any name given in mdadm.conf or any name information
stored in the metadata. If this name suggests a unit number, that
number will be used, otherwise a free unit number will be chosen.
Normally mdadm will prefer to create a partitionable array, however
if the CREATE line in mdadm.conf suggests that a non-partitionable
array is preferred, that will be honoured.
If the array is not found in the config file and its metadata does
not identify it as belonging to the "homehost", then mdadm will
choose a name for the array which is certain not to conflict with any
array which does belong to this host. It does this be adding an
underscore and a small number to the name preferred by the metadata.
Once an appropriate array is found or created and the device is
added, mdadm must decide if the array is ready to be started. It
will normally compare the number of available (non-spare) devices to
the number of devices that the metadata suggests need to be active.
If there are at least that many, the array will be started. This
means that if any devices are missing the array will not be
restarted.
As an alternative, --run may be passed to mdadm in which case the
array will be run as soon as there are enough devices present for the
data to be accessible. For a RAID1, that means one device will start
the array. For a clean RAID5, the array will be started as soon as
all but one drive is present.
Note that neither of these approaches is really ideal. If it can be
known that all device discovery has completed, then
mdadm -IRs
can be run which will try to start all arrays that are being
incrementally assembled. They are started in "read-auto" mode in
which they are read-only until the first write request. This means
that no metadata updates are made and no attempt at resync or
recovery happens. Further devices that are found before the first
write can still be added safely.
This section describes environment variables that affect how mdadm
operates.
MDADM_NO_MDMON
Setting this value to 1 will prevent mdadm from automatically
launching mdmon. This variable is intended primarily for
debugging mdadm/mdmon.
MDADM_NO_UDEV
Normally, mdadm does not create any device nodes in /dev, but
leaves that task to udev. If udev appears not to be
configured, or if this environment variable is set to '1', the
mdadm will create and devices that are needed.
MDADM_NO_SYSTEMCTL
If mdadm detects that systemd is in use it will normally
request systemd to start various background tasks
(particularly mdmon) rather than forking and running them in
the background. This can be suppressed by setting
MDADM_NO_SYSTEMCTL=1.
IMSM_NO_PLATFORM
A key value of IMSM metadata is that it allows
interoperability with boot ROMs on Intel platforms, and with
other major operating systems. Consequently, mdadm will only
allow an IMSM array to be created or modified if detects that
it is running on an Intel platform which supports IMSM, and
supports the particular configuration of IMSM that is being
requested (some functionality requires newer OROM support).
These checks can be suppressed by setting IMSM_NO_PLATFORM=1
in the environment. This can be useful for testing or for
disaster recovery. You should be aware that interoperability
may be compromised by setting this value.
MDADM_GROW_ALLOW_OLD
If an array is stopped while it is performing a reshape and
that reshape was making use of a backup file, then when the
array is re-assembled mdadm will sometimes complain that the
backup file is too old. If this happens and you are certain
it is the right backup file, you can over-ride this check by
setting MDADM_GROW_ALLOW_OLD=1 in the environment.
MDADM_CONF_AUTO
Any string given in this variable is added to the start of the
AUTO line in the config file, or treated as the whole AUTO
line if none is given. It can be used to disable certain
metadata types when mdadm is called from a boot script. For
example
export MDADM_CONF_AUTO='-ddf -imsm'
will make sure that mdadm does not automatically assemble any
DDF or IMSM arrays that are found. This can be useful on
systems configured to manage such arrays with dmraid.
mdadm --query /dev/name-of-device
This will find out if a given device is a RAID array, or is part of
one, and will provide brief information about the device.
mdadm --assemble --scan
This will assemble and start all arrays listed in the standard config
file. This command will typically go in a system startup file.
mdadm --stop --scan
This will shut down all arrays that can be shut down (i.e. are not
currently in use). This will typically go in a system shutdown
script.
mdadm --follow --scan --delay=120
If (and only if) there is an Email address or program given in the
standard config file, then monitor the status of all arrays listed in
that file by polling them ever 2 minutes.
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1
Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and
/dev/hdc1.
echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf
mdadm --detail --scan >> mdadm.conf
This will create a prototype config file that describes currently
active arrays that are known to be made from partitions of IDE or
SCSI drives. This file should be reviewed before being used as it
may contain unwanted detail.
echo 'DEVICE /dev/hd[a-z] /dev/sd*[a-z]' > mdadm.conf
mdadm --examine --scan --config=mdadm.conf >> mdadm.conf
This will find arrays which could be assembled from existing IDE and
SCSI whole drives (not partitions), and store the information in the
format of a config file. This file is very likely to contain
unwanted detail, particularly the devices= entries. It should be
reviewed and edited before being used as an actual config file.
mdadm --examine --brief --scan --config=partitions
mdadm -Ebsc partitions
Create a list of devices by reading /proc/partitions, scan these for
RAID superblocks, and printout a brief listing of all that were
found.
mdadm -Ac partitions -m 0 /dev/md0
Scan all partitions and devices listed in /proc/partitions and
assemble /dev/md0 out of all such devices with a RAID superblock with
a minor number of 0.
mdadm --monitor --scan --daemonise > /run/mdadm/mon.pid
If config file contains a mail address or alert program, run mdadm in
the background in monitor mode monitoring all md devices. Also write
pid of mdadm daemon to /run/mdadm/mon.pid.
mdadm -Iq /dev/somedevice
Try to incorporate newly discovered device into some array as
appropriate.
mdadm --incremental --rebuild-map --run --scan
Rebuild the array map from any current arrays, and then start any
that can be started.
mdadm /dev/md4 --fail detached --remove detached
Any devices which are components of /dev/md4 will be marked as faulty
and then remove from the array.
mdadm --grow /dev/md4 --level=6 --backup-file=/root/backup-md4
The array /dev/md4 which is currently a RAID5 array will be converted
to RAID6. There should normally already be a spare drive attached to
the array as a RAID6 needs one more drive than a matching RAID5.
mdadm --create /dev/md/ddf --metadata=ddf --raid-disks 6 /dev/sd[a-
f]
Create a DDF array over 6 devices.
mdadm --create /dev/md/home -n3 -l5 -z 30000000 /dev/md/ddf
Create a RAID5 array over any 3 devices in the given DDF set. Use
only 30 gigabytes of each device.
mdadm -A /dev/md/ddf1 /dev/sd[a-f]
Assemble a pre-exist ddf array.
mdadm -I /dev/md/ddf1
Assemble all arrays contained in the ddf array, assigning names as
appropriate.
mdadm --create --help
Provide help about the Create mode.
mdadm --config --help
Provide help about the format of the config file.
mdadm --help
Provide general help.
/proc/mdstat
If you're using the /proc filesystem, /proc/mdstat lists all active
md devices with information about them. mdadm uses this to find
arrays when --scan is given in Misc mode, and to monitor array
reconstruction on Monitor mode.
/etc/mdadm.conf
The config file lists which devices may be scanned to see if they
contain MD super block, and gives identifying information (e.g. UUID)
about known MD arrays. See mdadm.conf(5) for more details.
/etc/mdadm.conf.d
A directory containing configuration files which are read in lexical
order.
/run/mdadm/map
When --incremental mode is used, this file gets a list of arrays
currently being created.
mdadm understand two sorts of names for array devices.
The first is the so-called 'standard' format name, which matches the
names used by the kernel and which appear in /proc/mdstat.
The second sort can be freely chosen, but must reside in /dev/md/.
When giving a device name to mdadm to create or assemble an array,
either full path name such as /dev/md0 or /dev/md/home can be given,
or just the suffix of the second sort of name, such as home can be
given.
When mdadm chooses device names during auto-assembly or incremental
assembly, it will sometimes add a small sequence number to the end of
the name to avoid conflicted between multiple arrays that have the
same name. If mdadm can reasonably determine that the array really
is meant for this host, either by a hostname in the metadata, or by
the presence of the array in mdadm.conf, then it will leave off the
suffix if possible. Also if the homehost is specified as <ignore>
mdadm will only use a suffix if a different array of the same name
already exists or is listed in the config file.
The standard names for non-partitioned arrays (the only sort of md
array available in 2.4 and earlier) are of the form
/dev/mdNN
where NN is a number. The standard names for partitionable arrays
(as available from 2.6 onwards) are of the form:
/dev/md_dNN
Partition numbers should be indicated by adding "pMM" to these, thus
"/dev/md/d1p2".
From kernel version 2.6.28 the "non-partitioned array" can actually
be partitioned. So the "md_dNN" names are no longer needed, and
partitions such as "/dev/mdNNpXX" are possible.
From kernel version 2.6.29 standard names can be non-numeric
following the form:
/dev/md_XXX
where XXX is any string. These names are supported by mdadm since
version 3.3 provided they are enabled in mdadm.conf.
mdadm was previously known as mdctl.
For further information on mdadm usage, MD and the various levels of
RAID, see:
http://raid.wiki.kernel.org/
(based upon Jakob Østergaard's Software-RAID.HOWTO)
The latest version of mdadm should always be available from
http://www.kernel.org/pub/linux/utils/raid/mdadm/
Related man pages:
mdmon(8), mdadm.conf(5), md(4).
This page is part of the mdadm (Tool for managing md arrays in Linux)
project. Information about the project can be found at
⟨http://neil.brown.name/blog/mdadm⟩. If you have a bug report for
this manual page, send it to linux-raid@vger.kernl.org. This page
was obtained from the project's upstream Git repository
⟨https://github.com/neilbrown/mdadm.git⟩ on 2018-02-02. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2017-10-02.) If you discover any rendering problems in
this HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not part
of the original manual page), send a mail to man-pages@man7.org
v4.0 MDADM(8)
Pages that refer to this page: md(4), mdadm.conf(5), blkdeactivate(8), mdmon(8), raid6check(8)
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HTML rendering created 2018-02-02 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH.
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