Improve allocation and use of space for bitmaps in version1 metadata

[thirdparty/mdadm.git] / mdadm.8

.\" -*- nroff -*-
''' Copyright Neil Brown and others.
'''   This program is free software; you can redistribute it and/or modify
'''   it under the terms of the GNU General Public License as published by
'''   the Free Software Foundation; either version 2 of the License, or
'''   (at your option) any later version.
''' See file COPYING in distribution for details.
.TH MDADM 8 "" v2.5.6
.SH NAME
mdadm \- manage MD devices
.I aka
Linux Software Raid.

.SH SYNOPSIS

.BI mdadm " [mode] <raiddevice> [options] <component-devices>"

.SH DESCRIPTION
RAID devices are virtual devices created from two or more
real block devices. This allows multiple devices (typically disk
drives or partitions there-of) 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
.B LINEAR
md devices,
.B RAID0
(striping),
.B RAID1
(mirroring),
.BR RAID4 ,
.BR RAID5 ,
.BR RAID6 ,
.BR RAID10 ,
.BR MULTIPATH ,
and
.BR FAULTY .

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

.B 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.

'''.B mdadm
'''is a program that can be used to create, manage, and monitor
'''MD devices.  As
'''such it provides a similar set of functionality to the
'''.B raidtools
'''packages.
'''The key differences between
'''.B mdadm
'''and
'''.B raidtools
'''are:
'''.IP \(bu 4
'''.B mdadm
'''is a single program and not a collection of programs.
'''.IP \(bu 4
'''.B mdadm
'''can perform (almost) all of its functions without having a
'''configuration file and does not use one by default.  Also
'''.B mdadm
'''helps with management of the configuration
'''file.
'''.IP \(bu 4
'''.B mdadm
'''can provide information about your arrays (through Query, Detail, and Examine)
'''that
'''.B  raidtools
'''cannot.
'''.P
'''.I mdadm
'''does not use
'''.IR /etc/raidtab ,
'''the
'''.B raidtools
'''configuration file, at all.  It has a different configuration file
'''with a different format and a different purpose.

.SH MODES
mdadm has 7 major modes of operation:
.TP
.B Assemble
Assemble the parts of a previously created
array into an active array. Components can be explicitly given
or can be searched for.
.B 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.

.TP
.B Build
Build an array that doesn't have per-device superblocks.  For these
sorts of arrays,
.I mdadm
cannot differentiate between initial creation and subsequent assembly
of an array.  It also cannot perform any checks that appropriate
devices have been requested.  Because of this, the
.B Build
mode should only be used together with a complete understanding of
what you are doing.

.TP
.B Create
Create a new array with per-device superblocks.
'''It can progress
'''in several step create-add-add-run or it can all happen with one command.

.TP
.B "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.

.TP
.B "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 in RAID level 1/4/5/6 and changing the number of
active devices in RAID1.

.TP
.B Manage
This is for doing things to specific components of an array such as
adding new spares and removing faulty devices.

.TP
.B 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.
'''This mode allows operations on independent devices such as examine MD
'''superblocks, erasing old superblocks and stopping active arrays.

.SH OPTIONS

.SH Options for selecting a mode are:

.TP
.BR -A ", " --assemble
Assemble a pre-existing array.

.TP
.BR -B ", " --build
Build a legacy array without superblocks.

.TP
.BR -C ", " --create
Create a new array.

.TP
.BR -F ", " --follow ", " --monitor
Select
.B Monitor
mode.

.TP
.BR -G ", " --grow
Change the size or shape of an active array.
.P
If a device is given before any options, or if the first option is
.BR --add ,
.BR --fail ,
or
.BR --remove ,
then the MANAGE mode is assume.
Anything other than these will cause the
.B Misc
mode to be assumed.

.SH Options that are not mode-specific are:

.TP
.BR -h ", " --help
Display general help message or, after one of the above options, a
mode specific help message.

.TP
.B --help-options
Display more detailed help about command line parsing and some commonly
used options.

.TP
.BR -V ", " --version
Print version information for mdadm.

.TP
.BR -v ", " --verbose
Be more verbose about what is happening.  This can be used twice to be
extra-verbose.
The extra verbosity currently only affects
.B --detail --scan
and
.BR "--examine --scan" .

.TP
.BR -q ", " --quiet
Avoid printing purely informative messages.  With this,
.B mdadm
will be silent unless there is something really important to report.

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

.TP
.BR -f ", " --force
Be more forceful about certain operations.  See the various modes of
the exact meaning of this option in different contexts.

.TP
.BR -c ", " --config=
Specify the config file.  Default is to use
.BR /etc/mdadm.conf ,
or if that is missing, then
.BR /etc/mdadm/mdadm.conf .
If the config file given is
.B partitions
then nothing will be read, but
.I mdadm
will act as though the config file contained exactly
.B "DEVICE partitions"
and will read
.B /proc/partitions
to find a list of devices to scan.
If the word
.B none
is given for the config file, then
.I mdadm
will act as though the config file were empty.

.TP
.BR -s ", " --scan
scan config file or
.B /proc/mdstat
for missing information.
In general, this option gives
.B mdadm
permission to get any missing information, like component devices,
array devices, array identities, and alert destination from the
configuration file:
.BR /etc/mdadm.conf .
One exception is MISC mode when using
.B --detail
or
.B --stop
in which case
.B --scan
says to get a list of array devices from
.BR /proc/mdstat .

.TP
.B -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
.B metadata
value for the
.B CREATE
keyword in
.BR mdadm.conf .

Options are:
.RS
.IP "0, 0.90, default"
Use the original 0.90 format superblock.  This format limits arrays to
28 componenet devices and limits component devices of levels 1 and
greater to 2 terabytes.
.IP "1, 1.0, 1.1, 1.2"
Use the new version-1 format superblock.  This has few restrictions.
The different subversion 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).
.RE

.TP
.B --homehost=
This will over-ride any
.B HOMEHOST
setting in the config file and provides the identify of the host which
should be considered the home for any arrays.

When creating an array, the
.B homehost
will be recorded in the superblock.  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 assembled.

.SH For create, build, or grow:

.TP
.BR -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
.I component-devices
(including "\fBmissing\fP" devices)
that are listed on the command line for
.BR  --create .
Setting a value of 1 is probably
a mistake and so requires that
.B --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.
.br
This number can only be changed using
.B --grow
for RAID1 arrays, and only on kernels which provide necessary support.

.TP
.BR -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.


.TP
.BR -z ", " --size=
Amount (in Kibibytes) of space to use from each drive in RAID1/4/5/6.
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
.B --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
.BR --grow .
The size can be given as
.B max
which means to choose the largest size that fits on all current drives.

.TP
.BR -c ", " --chunk=
Specify chunk size of kibibytes.  The default is 64.

.TP
.BR --rounding=
Specify rounding factor for linear array (==chunk size)

.TP
.BR -l ", " --level=
Set raid level.  When used with
.IR --create ,
options are: linear, raid0, 0, stripe, raid1, 1, mirror, raid4, 4,
raid5, 5, raid6, 6, raid10, 10, multipath, mp, faulty.  Obviously some of these are synonymous.

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

Not yet supported with
.IR --grow .

.TP
.BR -p ", " --layout=
This option configures the fine details of data layout for raid5,
and raid10 arrays, and controls the failure modes for
.IR 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
.I 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 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 request, 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 'p' followed
by a small number.  The default is 'n2'.

.I n
signals 'near' copies. Multiple copies of one data block are at
similar offsets in different devices.

.I 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.

.I 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).

.TP
.BR --parity=
same as --layout (thus explaining the p of
.IR -p ).

.TP
.BR -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
.B internal
is given, then the bitmap is stored with the metadata on the array,
and so is replicated on all devices.  If the word
.B none
is given with
.B --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.

.TP
.BR --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 atleast 4 and requires no more than 2^21 chunks.
When using an
.B internal
bitmap, the chunksize is automatically determined to make best use of
available space.


.TP
.BR -W ", " --write-mostly
subsequent devices lists in a
.BR --build ,
.BR --create ,
or
.B --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.

.TP
.BR --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
.IR write-mostly .

.TP
.BR --assume-clean
Tell
.I 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 ony if you really know what you are doing.

.TP
.BR --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.

.TP
.BR -N ", " --name=
Set a
.B 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.

.TP
.BR -R ", " --run
Insist that
.I mdadm
run the array, even if some of the components
appear to be active in another array or filesystem.  Normally
.I mdadm
will ask for confirmation before including such components in an
array.  This option causes that question to be suppressed.

.TP
.BR -f ", " --force
Insist that
.I mdadm
accept the geometry and layout specified without question.  Normally
.I 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
.BR --force ,
.I mdadm
will not try to be so clever.

.TP
.BR -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
.I --scan
is also given, then any
.I auto=
entries in the config file will over-ride the
.I --auto
instruction given on the command line.

For partitionable arrays,
.I 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.
.BR --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.

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


.SH For assemble:

.TP
.BR -u ", " --uuid=
uuid of array to assemble. Devices which don't have this uuid are
excluded

.TP
.BR -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
.B --super-minor
will cause
.I mdadm
to use the minor number of the md device that is being assembled.
e.g. when assembling
.BR /dev/md0 ,
.M --super-minor=dev
will look for super blocks with a minor number of 0.

.TP
.BR -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
then name stored in the superblock exactly, or it must match
with the current
.I homehost
is added to the start of the given name.

.TP
.BR -f ", " --force
Assemble the array even if some superblocks appear out-of-date

.TP
.BR -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
.B --scan
is not used, then the array will be assembled but not started.
With
.B --run
an attempt will be made to start it anyway.

.TP
.B --no-degraded
This is the reverse of
.B --run
in that it inhibits the started if array unless all expected drives
are present.  This is only needed with
.B --scan
and can be used if you physical connections to devices are
not as reliable as you would like.

.TP
.BR -a ", " "--auto{=no,yes,md,mdp,part}"
See this option under Create and Build options.

.TP
.BR -b ", " --bitmap=
Specify the bitmap file that was given when the array was created.  If
an array has an
.B internal
bitmap, there is no need to specify this when assembling the array.

.TP
.BR --backup-file=
If
.B --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
.B --backup-file
must be presented to --assemble to allow possibly corrupted data to be
restored.

.TP
.BR -U ", " --update=
Update the superblock on each device while assembling the array.  The
argument given to this flag can be one of
.BR sparc2.2 ,
.BR summaries ,
.BR uuid ,
.BR name ,
.BR homehost ,
.BR resync ,
.BR byteorder ,
or
.BR super-minor .

The
.B 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
.B "--examine --sparc2.2"
option to
.I mdadm
to see what effect this would have.

The
.B super-minor
option will update the
.B "preferred minor"
field on each superblock to match the minor number of the array being
assembled.
This can be useful if
.B --examine
reports a different "Preferred Minor" to
.BR --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
.B 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
.B NOT
be used to help identify the devices in the array.
If no "--uuid" is given, a random uuid is chosen.

The
.B name
option will change the
.I name
of the array as stored in the superblock.  This is only supported for
version-1 superblocks.

The
.B homehost
option will change the
.I 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
.B resync
option will cause the array to be marked
.I 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
.B 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
.B "--update=byteorder"
will cause
.I 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
.B summaries
option will correct the summaries in the superblock. That is the
counts of total, working, active, failed, and spare devices.

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

.SH For Manage mode:

.TP
.BR -a ", " --add
hot-add listed devices.

.TP
.BR --re-add
re-add a device that was recently removed from an array.

.TP
.BR -r ", " --remove
remove listed devices.  They must not be active.  i.e. they should
be failed or spare devices.

.TP
.BR -f ", " --fail
mark listed devices as faulty.

.TP
.BR --set-faulty
same as --fail.

.P
Each of these options require that the first device list is the array
to be acted upon and the remainder are component devices to be added,
removed, or marked as fault.  Several different operations can be
specified for different devices, e.g.
.in +5
mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove /dev/sdb1
.in -5
Each operation applies to all devices listed until the next
operations.

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 updated the blocks that have changed
since the device was removed.  For arrays with persistent metadata
(superblocks) this is done automatically.  For arrays created with
.B --build
mdadm needs to be told that this device we removed recently with
.B --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 be marked as
.B faulty
first.

.SH For Misc mode:

.TP
.BR -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.

.TP
.BR -D ", " --detail
Print detail of one or more md devices.

.TP
.BR -E ", " --examine
Print content of md superblock on device(s).
.TP
.B --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
.B --sparc2.2
flag with
.B --examine
will fix the superblock before displaying it.  If this appears to do
the right thing, then the array can be successfully assembled using
.BR "--assemble --update=sparc2.2" .

.TP
.BR -X ", " --examine-bitmap
Report information about a bitmap file.

.TP
.BR -R ", " --run
start a partially built array.

.TP
.BR -S ", " --stop
deactivate array, releasing all resources.

.TP
.BR -o ", " --readonly
mark array as readonly.

.TP
.BR -w ", " --readwrite
mark array as readwrite.

.TP
.B --zero-superblock
If the device contains a valid md superblock, the block is
over-written with zeros.  With
--force
the block where the superblock would be is over-written even if it
doesn't appear to be valid.

.TP
.BR -t ", " --test
When used with
.BR --detail ,
the exit status of
.I mdadm
is set to reflect the status of the device.

.SH For Monitor mode:
.TP
.BR -m ", " --mail
Give a mail address to send alerts to.

.TP
.BR -p ", " --program ", " --alert
Give a program to be run whenever an event is detected.

.TP
.BR -y ", " --syslog
Cause all events to be reported through 'syslog'.  The messages have
facility of 'daemon' and varying priorities.

.TP
.BR -d ", " --delay
Give a delay in seconds.
.B mdadm
polls the md arrays and then waits this many seconds before polling
again.  The default is 60 seconds.

.TP
.BR -f ", " --daemonise
Tell
.B 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
.B --scan
which will only continue monitoring if a mail address or alert program
is found in the config file.

.TP
.BR -i ", " --pid-file
When
.B mdadm
is running in daemon mode, write the pid of the daemon process to
the specified file, instead of printing it on standard output.

.TP
.BR -1 ", " --oneshot
Check arrays only once.  This will generate
.B NewArray
events and more significantly
.B DegradedArray
and
.B SparesMissing
events.  Running
.in +5
.B "   mdadm --monitor --scan -1"
.in -5
from a cron script will ensure regular notification of any degraded arrays.

.TP
.BR -t ", " --test
Generate a
.B 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.

.SH ASSEMBLE MODE

.HP 12
Usage:
.B mdadm --assemble
.I md-device options-and-component-devices...
.HP 12
Usage:
.B mdadm --assemble --scan
.I  md-devices-and-options...
.HP 12
Usage:
.B mdadm --assemble --scan
.I  options...

.PP
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
.BR --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 precisely one device is listed, but
.B --scan
is not given, then
.I mdadm
acts as though
.B --scan
was given and identify information is extracted from the configuration file.

The identity can be given with the
.B --uuid
option, with the
.B --super-minor
option, can be found  in the config file, or will be taken from the
super block on the first component-device listed on the command line.

Devices can be given on the
.B --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
.B --config
or requested with (a possibly implicit)
.B --scan.
In the later case,
.B /etc/mdadm.conf
is used.

If
.B --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
.B --scan
is not given and insufficient drives were listed to start a complete
(non-degraded) array, 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
.B --run
flag.

If an
.B auto
option is given, either on the command line (--auto) or in the
configuration file (e.g. auto=part), then
.I mdadm
will create the md device if necessary or will re-create it if it
doesn't look usable as it is.

This can be useful for handling partitioned devices (which don't have
a stable device number - it can change after a reboot) and when using
"udev" to manage your
.B /dev
tree (udev cannot handle md devices because of the unusual device
initialisation conventions).

If the option to "auto" is "mdp" or "part" or (on the command line
only) "p", then mdadm will create a partitionable array, using the
first free one that is not in use, and does not already have an entry
in /dev (apart from numeric /dev/md* entries).

If the option to "auto" is "yes" or "md" or (on the command line)
nothing, then mdadm will create a traditional, non-partitionable md
array.

It is expected that the "auto" functionality will be used to create
device entries with meaningful names such as "/dev/md/home" or
"/dev/md/root", rather than names based on the numerical array number.

When using this option to create a partitionable array, the device
files for the first 4 partitions are also created. If a different
number is required it can be simply appended to the auto option.
e.g. "auto=part8".  Partition names are created by appending a digit
string to the device name, with an intervening "p" if the device name
ends with a digit.

The
.B --auto
option is also available in Build and Create modes.  As those modes do
not use a config file, the "auto=" config option does not apply to
these modes.

.SS Auto Assembly
When
.B --assemble
is used with
.B --scan
and no devices are listed,
.I mdadm
will first attempt to assemble all the arrays listed in the config
file.

If a
.B homehost
has been specified (either in the config file or on the command line),
.I mdadm
will look further for possible arrays and will try to assemble
anything that it finds which is tagged as belonging to the given
homehost.  This is the only situation where
.I mdadm
will assemble arrays without being given specific device name or
identify information for the array.

If
.I 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
.B minor
number as recorded in the superblock is used to create a name in
.B /dev/md/
so for example
.BR /dev/md/3 .
If the array uses version-1 metadata, then the
.B name
from the superblock is used to similarly create a name in
.BR /dev/md .
The name will have any 'host' prefix stripped first.

If
.I mdadm
cannot find any array for the given host at all, and if
.B --auto-update-homehost
is given, then
.I mdadm
will search again for any array (not just an array created for this
host) and will assemble each assuming
.IR --update=homehost .
This will change the host tag in the superblock so that on the next run,
these arrays will be found without the second pass.  The intention of
this feature is to support transitioning a set of md arrays to using
homehost tagging.

The reason for requiring arrays to be tagged with the homehost for
auto assembly is to guard against problems that can arise when moving
devices from one host to another.

.SH BUILD MODE

.HP 12
Usage:
.B mdadm --build
.I device
.BI --chunk= X
.BI --level= Y
.BI --raid-devices= Z
.I devices

.PP
This usage is similar to
.BR --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, multipath, or faulty, or one of their
synonyms. All devices must be listed and the array will be started
once complete.

.SH CREATE MODE

.HP 12
Usage:
.B mdadm --create
.I device
.BI --chunk= X
.BI --level= Y
.br
.BI --raid-devices= Z
.I  devices

.PP
This usage will initialise a new md array, associate some devices with
it, and activate the array.

If the
.B --auto
option is given (as described in more detail in the section on
Assemble mode), then the md device will be created with a suitable
device number if necessary.

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
.B --run
can override this caution.

To create a "degraded" array in which some devices are missing, simply
give the word "\fBmissing\fP"
in place of a device name.  This will cause
.B mdadm
to leave the corresponding slot in the array empty.
For a RAID4 or RAID5 array at most one slot can be
"\fBmissing\fP"; 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
"\fBmissing\fP".

When creating a RAID5 array,
.B 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 over-ridden with the
.I --force
option.

When creating an array with version-1 metadata a name for the host is
required.
If this is not given with the
.B --name
option,
.I mdadm
will chose a name based on the last component of the name of the
device being created.  So if
.B /dev/md3
is being created, then the name
.B 3
will be chosen.
If
.B /dev/md/home
is being created, then the name
.B home
will be used.

'''If the
'''.B --size
'''option is given, it is not necessary to list any component-devices in this command.
'''They can be added later, before a
'''.B --run.
'''If no
'''.B --size
'''is given, the apparent size of the smallest drive given is used.

The General Management options that are valid with --create are:
.TP
.B --run
insist on running the array even if some devices look like they might
be in use.

.TP
.B --readonly
start the array readonly - not supported yet.


.SH MANAGE MODE
.HP 12
Usage:
.B mdadm
.I device
.I options... devices...
.PP

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:
.br
.B "  mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1"
.br
will firstly mark
.B /dev/hda1
as faulty in
.B /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.

.SH MISC MODE
.HP 12
Usage:
.B mdadm
.I options ...
.I devices  ...
.PP

MISC mode includes a number of distinct operations that
operate on distinct devices.  The operations are:
.TP
--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.

.TP
--detail
The device should be an active md device.
.B   mdadm
will display a detailed description of the array.
.B --brief
or
.B --scan
will cause the output to be less detailed and the format to be
suitable for inclusion in
.BR /etc/mdadm.conf .
The exit status of
.I mdadm
will normally be 0 unless
.I mdadm
failed to get useful information about the device(s).  However if the
.B --test
option is given, then the exit status will be:
.RS
.TP
0
The array is functioning normally.
.TP
1
The array has at least one failed device.
.TP
2
The array has multiple failed devices and hence is unusable (raid4 or
raid5).
.TP
4
There was an error while trying to get information about the device.
.RE

.TP
--examine
The device should be a component of an md array.
.B mdadm
will read the md superblock of the device and display the contents.
If
.B --brief
is given, or
.B --scan
then multiple devices that are components of the one array
are grouped together and reported in a single entry suitable
for inclusion in
.BR /etc/mdadm.conf .

Having
.B --scan
without listing any devices will cause all devices listed in the
config file to be examined.

.TP
--stop
The devices should be active md arrays which will be deactivated, as
long as they are not currently in use.

.TP
--run
This will fully activate a partially assembled md array.

.TP
--readonly
This will mark an active array as read-only, providing that it is
not currently being used.

.TP
--readwrite
This will change a
.B readonly
array back to being read/write.

.TP
--scan
For all operations except
.BR --examine ,
.B --scan
will cause the operation to be applied to all arrays listed in
.BR /proc/mdstat .
For
.BR --examine,
.B --scan
causes all devices listed in the config file to be examined.


.SH MONITOR MODE

.HP 12
Usage:
.B mdadm --monitor
.I options... devices...

.PP
This usage causes
.B mdadm
to periodically poll a number of md arrays and to report on any events
noticed.
.B 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,
.B mdadm
may move a spare drive from one array to another if they are in the
same
.B spare-group
and if the destination array has a failed drive but no spares.

If any devices are listed on the command line,
.B mdadm
will only monitor those devices. Otherwise all arrays listed in the
configuration file will be monitored.  Further, if
.B --scan
is given, then any other md devices that appear in
.B /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 event to 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
.B --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
.B mdadm
will not monitor anything.
Without
.B --scan
.B 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
.BR stdout .

The different events are:

.RS 4
.TP
.B DeviceDisappeared
An md array which previously was configured appears to no longer be
configured. (syslog priority: Critical)

If
.I mdadm
was told to monitor an array which is RAID0 or Linear, then it will
report
.B DeviceDisappeared
with the extra information
.BR Wrong-Level .
This is because RAID0 and Linear do not support the device-failed,
hot-spare and resync operations which are monitored.

.TP
.B RebuildStarted
An md array started reconstruction. (syslog priority: Warning)

.TP
.BI Rebuild NN
Where
.I NN
is 20, 40, 60, or 80, this indicates that rebuild has passed that many
percentage of the total. (syslog priority: Warning)

.TP
.B RebuildFinished
An md array that was rebuilding, isn't any more, either because it
finished normally or was aborted. (syslog priority: Warning)

.TP
.B Fail
An active component device of an array has been marked as
faulty. (syslog priority: Critical)

.TP
.B FailSpare
A spare component device which was being rebuilt to replace a faulty
device has failed. (syslog priority: Critial)

.TP
.B 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)

.TP
.B NewArray
A new md array has been detected in the
.B /proc/mdstat
file.   (syslog priority: Info)

.TP
.B DegradedArray
A newly noticed array appears to be degraded.  This message is not
generated when
.I mdadm
notices a drive failure which causes degradation, but only when
.I mdadm
notices that an array is degraded when it first sees the array.
(syslog priority: Critial)

.TP
.B MoveSpare
A spare drive has been moved from one array in a
.B spare-group
to another to allow a failed drive to be replaced.
(syslog priority: Info)

.TP
.B SparesMissing
If
.I mdadm
has been told, via the config file, that an array should have a certain
number of spare devices, and
.I mdadm
detects that it has fewer that this number when it first sees the
array, it will report a
.B SparesMissing
message.
(syslog priority: Warning)

.TP
.B TestMessage
An array was found at startup, and the
.B --test
flag was given.
(syslog priority: Info)
.RE

Only
.B Fail ,
.B FailSpare ,
.B DegradedArray ,
.B SparesMissing ,
and
.B TestMessage
cause Email to be sent.  All events cause the program to be run.
The program is run with two or three arguments, they being the event
name, the array device and possibly a second device.

Each event has an associated array device (e.g.
.BR /dev/md1 )
and possibly a second device.  For
.BR Fail ,
.BR FailSpare ,
and
.B SpareActive
the second device is the relevant component device.
For
.B MoveSpare
the second device is the array that the spare was moved from.

For
.B mdadm
to move spares from one array to another, the different arrays need to
be labelled with the same
.B spare-group
in the configuration file.  The
.B spare-group
name can be any string. It is only necessary that different spare
groups use different names.

When
.B mdadm
detects that an array which is 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.

.SH GROW MODE
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,
including restructuring a raid5 array to have more active devices.

Currently the only support available is to
.IP \(bu 4
change the "size" attribute
for RAID1, RAID5 and RAID6.
.IP \(bu 4
increase the "raid-disks" attribute of RAID1 and RAID5.
.IP \(bu 4
add a write-intent bitmap to any array which support these bitmaps, or
remove a write-intent bitmap from such an array.
.PP

.SS SIZE CHANGES
Normally when an array is built the "size" it 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 to use the space.  The
filesystem will need to be explicitly told to use the extra space.

.SS 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 that 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.

Increasing the number of active devices in a RAID5 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 do
this safely, including restart and interrupted "reshape".

When relocating the first few stripes on a raid5, 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.  This backup is normally stored in any spare
devices that the array has, however it can also be stored in a
separate file specified with the
.B --backup-file
option.  If this option is used, and the system does crash during the
critical period, the same file must be passed to
.B --assemble
to restore the backup and reassemble the array.

.SS 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.

.SH EXAMPLES

.B "  mdadm --query /dev/name-of-device"
.br
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.

.B "  mdadm --assemble --scan"
.br
This will assemble and start all arrays listed in the standard config file
file.  This command will typically go in a system startup file.

.B "  mdadm --stop --scan"
.br
This will shut down all array that can be shut down (i.e. are not
currently in use).  This will typically go in a system shutdown script.

.B "  mdadm --follow --scan --delay=120"
.br
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.

.B "  mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1"
.br
Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and /dev/hdc1.

.br
.B "  echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf"
.br
.B "  mdadm --detail --scan >> mdadm.conf"
.br
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.

.B "  echo 'DEVICE /dev/hd[a-z] /dev/sd*[a-z]' > mdadm.conf"
.br
.B "  mdadm --examine --scan --config=mdadm.conf >> mdadm.conf"
.ber
This will find what arrays could be assembled from existing IDE and
SCSI whole drives (not partitions) and store the information is the
format of a config file.
This file is very likely to contain unwanted detail, particularly
the
.B devices=
entries.  It should be reviewed and edited before being used as an
actual config file.

.B "  mdadm --examine --brief --scan --config=partitions"
.br
.B "  mdadm -Ebsc partitions"
.br
Create a list of devices by reading
.BR /proc/partitions ,
scan these for RAID superblocks, and printout a brief listing of all
that was found.

.B "  mdadm -Ac partitions -m 0 /dev/md0"
.br
Scan all partitions and devices listed in
.BR /proc/partitions
and assemble
.B /dev/md0
out of all such devices with a RAID superblock with a minor number of 0.

.B "  mdadm --monitor --scan --daemonise > /var/run/mdadm"
.br
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
.BR /var/run/mdadm .

.B "  mdadm --create --help"
.br
Provide help about the Create mode.

.B "  mdadm --config --help"
.br
Provide help about the format of the config file.

.B "  mdadm --help"
.br
Provide general help.


.SH FILES

.SS /proc/mdstat

If you're using the
.B /proc
filesystem,
.B /proc/mdstat
lists all active md devices with information about them.
.B mdadm
uses this to find arrays when
.B --scan
is given in Misc mode, and to monitor array reconstruction
on Monitor mode.


.SS /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
.BR mdadm.conf (5)
for more details.

.SH DEVICE NAMES

While entries in the /dev directory can have any format you like,
.I mdadm
has an understanding of 'standard' formats which it uses to guide its
behaviour when creating device files via the
.I --auto
option.

The standard names for non-partitioned arrays (the only sort of md
array available in 2.4 and earlier) either of
.IP
/dev/mdNN
.br
/dev/md/NN
.PP
where NN is a number.
The standard names for partitionable arrays (as available from 2.6
onwards) is one of
.IP
/dev/md/dNN
.br
/dev/md_dNN
.PP
Partition numbers should be indicated by added "pMM" to these, thus "/dev/md/d1p2".

.SH NOTE
.B mdadm
was previously known as
.BR mdctl .
.P
.B mdadm
is completely separate from the
.B raidtools
package, and does not use the
.I /etc/raidtab
configuration file at all.

.SH SEE ALSO
For information on the various levels of
RAID, check out:

.IP
.UR   http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/
http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/
.UE
'''.PP
'''for new releases of the RAID driver check out:
'''
'''.IP
'''.UR  ftp://ftp.kernel.org/pub/linux/kernel/people/mingo/raid-patches
'''ftp://ftp.kernel.org/pub/linux/kernel/people/mingo/raid-patches
'''.UE
'''.PP
'''or
'''.IP
'''.UR http://www.cse.unsw.edu.au/~neilb/patches/linux-stable/
'''http://www.cse.unsw.edu.au/~neilb/patches/linux-stable/
'''.UE
.PP
The latest version of
.I mdadm
should always be available from
.IP
.UR http://www.kernel.org/pub/linux/utils/raid/mdadm/
http://www.kernel.org/pub/linux/utils/raid/mdadm/
.UE
.PP
.IR mdadm.conf (5),
.IR md (4).
.PP
.IR raidtab (5),
.IR raid0run (8),
.IR raidstop (8),
.IR mkraid (8).