man mdadm Command

Man page for apt-get mdadm Command

Man Page for mdadm in Linux

Ubuntu Man Command : man mdadm

Man Mdadm  Command

This tutorial shows the man page for man mdadm in linux.

Open terminal with 'su' access and type the command as shown below:
man mdadm

Result of the Command Execution shown below:

MDADM(8)                                                                                                                                                    MDADM(8)



NAME
mdadm manage MD devices aka Linux Software RAID


SYNOPSIS
mdadm [mode] <raiddevice> [options] <component devices>


DESCRIPTION
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, and FAULTY.

MULTIPATH is not a Software RAID mechanism, but does involve multiple devices: each device is a path to one common physical storage device.

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.



MODES
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 superblocks. For these sorts of arrays, mdadm cannot differentiate between initial creation and subse Äê
quent 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 superblocks.


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 RAID levels 1/4/5/6, as well as adding or removing a write intent bitmap.


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 con Äê
venient interface to a hot plug system. As each device is detected, mdadm has a chance to include it in some array as appropriate.


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.

OPTIONS
Options for selecting a mode are:
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 a single device into 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  Äî 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. In kernel autodetect is not recommended
for new installations. Using mdadm to detect and assemble arrays  Äî possibly in an initrd  Äî is substantially more flexible and should be preferred.


If a device is given before any options, or if the first option is add, fail, or remove, then the MANAGE mode is assume. Anything other than these
will cause the Misc mode to be assumed.


Options that are not mode specific are:
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.


b, brief
Be less verbose. This is used with detail and examine. Using brief with verbose gives an intermediate level of verbosity.


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. Default is to use /etc/mdadm/mdadm.conf, or if that is missing, then /etc/mdadm.conf. If the config file given is parti Äê
tions then nothing will be read, but mdadm will act as though the config file contained exactly DEVICE partitions and will read /proc/partitions to
find a list of devices to scan. If the word none is given for the config file, then mdadm will act as though the config file were empty.


s, scan
Scan config file or /proc/mdstat for missing information. In general, this option gives mdadm permission to get any missing information (like compo Äê
nent 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 superblock (raid metadata) to be used. The default is 0.90 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, default
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.

1, 1.0, 1.1, 1.2
Use the new version 1 format superblock. This has few restrictions. 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).


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 superblock. For version 1 superblocks, it will be prefixed to the array name. For ver Äê
sion 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 assembled.


For create, build, or grow:
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 or raid5.
This number can only be changed using grow for RAID1, RAID5 and RAID6 arrays, and only on kernels which provide 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 Kibibytes) of space to use from each drive in RAID level 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.

This value can be set with grow for RAID level 1/4/5/6. 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.


c, chunk=
Specify chunk size of kibibytes. The default is 64.


rounding=
Specify rounding factor for linear array (==chunk size)


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, multi Äê
path, mp, faulty. Obviously some of these are synonymous.

When used with build, only linear, stripe, raid0, 0, raid1, multipath, mp, and faulty are valid.

Not yet supported with grow.


p, layout=
This option configures the fine details of data layout for raid5, 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.

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.

To set the parity with grow, the level of the array ("faulty") must be specified before the fault mode is specified.

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' 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).


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.

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.


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 is automatically determined to
make best use of available space.



W, write mostly
subsequent devices lists 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.


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  Äî while normally safe  Äî is not recommended. Use this only if you really know what you are doing.


backup file=
This is needed when grow is used to increase the number of raid devices in a RAID5 if there are no spare devices available. See the section below
on RAID_DEVICE CHANGES. The file should be stored on a separate device, not on the raid array being reshaped.


N, name=
Set a name for the array. This is currently only effective when creating an array with a version 1 superblock. The name is a simple textual string
that can be used to identify array components when assembling.


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 con Äê
firmation 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.


a, auto{=no,yes,md,mdp,part,p}{NN}
Instruct mdadm to create the device file if needed, possibly allocating an unused minor number. "md" causes a non partitionable array to be used.
"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. 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/home1p3". If there is no trailing digit, then the partition names just have a number added, e.g. "/dev/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 number based
on that name. If the device name is not in one of these formats, then a unused minor number will be allocated. The minor 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.


symlink=no
Normally when auto causes mdadm to create devices in /dev/md/ it will also create symlinks from /dev/ with names starting with md or md_. Use
symlink=no to suppress this, or symlink=yes to enforce this even if it is suppressing mdadm.conf.



For assemble:
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 assem Äê
bling /dev/md0, super minor=dev will look for super blocks with a minor number of 0.


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 some superblocks appear out of date


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 assem Äê
bling the array.


backup file=
If backup file was used to grow the number of raid devices in a RAID5, 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.


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, home Äê
host, resync, byteorder, devicesize, 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 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 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. 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 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.


auto update homehost
This flag is only meaningful with auto assembly (see discussion below). In that situation, if no suitable arrays are found for this homehost, mdadm
will rescan for any arrays at all and will assemble them and update the homehost to match the current host.


For Manage mode:
a, add
hot add listed devices.


re add
re add a device that was recently removed from an 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 and detached 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. This will only succeed for devices that are spares or have already been
marked as failed.


f, fail
mark listed devices as faulty. As well as the name of a device file, the word detached can be given. This will cause any device that has been
detached from the system to be marked as failed. It can then be removed.


set faulty
same as fail.


Each of these options require that the first device listed is the array to be acted upon, and the remainder are component devices to be added, removed, or
marked as faulty. 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.


For Misc mode:
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 detail of one or more md devices.


Y, export
When used with detail or examine, output will be formatted as key=value pairs for easy import into the environment.


E, examine
Print content of md superblock on device(s).

sparc2.2
If an array was created on 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.


R, run
start a partially built array.


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 overwrit Äê
ten even if it doesn't appear to be valid.


t, test
When used with detail, the exit status of mdadm is set to reflect the status of the device.


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 actu Äê
ally waited for every device listed, otherwise it will return failure.


For Incremental Assembly mode:
rebuild map, r
Rebuild the map file (/var/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.


For Monitor mode:
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.


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 form 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.


ASSEMBLE MODE
Usage: mdadm assemble md device options and component devices...

Usage: md


Related Topics

Apt Get Commands