mdadm is a tool for creating, managing, and monitoring RAID devices
using the md driver in Linux.
Some common tasks, such as assembling all arrays, can be simplified
by describing the devices and arrays in this configuration file.
The file should be seen as a collection of words separated by white
space (space, tab, or newline). Any word that beings with a hash
sign (#) starts a comment and that word together with the remainder
of the line is ignored.
Spaces can be included in a word using quotation characters. Either
single quotes (') or double quotes (") may be used. All the
characters from one quotation character to next identical character
are protected and will not be used to separate words to start new
quoted strings. To include a single quote it must be between double
quotes. To include a double quote it must be between single quotes.
Any line that starts with white space (space or tab) is treated as
though it were a continuation of the previous line.
Empty lines are ignored, but otherwise each (non continuation) line
must start with a keyword as listed below. The keywords are case
insensitive and can be abbreviated to 3 characters.
The keywords are:
DEVICE A device line lists the devices (whole devices or partitions)
that might contain a component of an MD array. When looking
for the components of an array, mdadm will scan these devices
(or any devices listed on the command line).
The device line may contain a number of different devices
(separated by spaces) and each device name can contain wild
cards as defined by glob(7).
Also, there may be several device lines present in the file.
Alternatively, a device line can contain either or both of the
words containers and partitions. The word containers will
cause mdadm to look for assembled CONTAINER arrays and
included them as a source for assembling further arrays.
The word partitions will cause mdadm to read /proc/partitions
and include all devices and partitions found therein. mdadm
does not use the names from /proc/partitions but only the
major and minor device numbers. It scans /dev to find the
name that matches the numbers.
If no DEVICE line is present, then "DEVICE partitions
containers" is assumed.
DEVICE /dev/hda* /dev/hdc*
ARRAY The ARRAY lines identify actual arrays. The second word on
the line may be the name of the device where the array is
normally assembled, such as /dev/md1 or /dev/md/backup. If
the name does not start with a slash ('/'), it is treated as
being in /dev/md/. Alternately the word <ignore> (complete
with angle brackets) can be given in which case any array
which matches the rest of the line will never be automatically
assembled. If no device name is given, mdadm will use various
heuristics to determine an appropriate name.
Subsequent words identify the array, or identify the array as
a member of a group. If multiple identities are given, then a
component device must match ALL identities to be considered a
match. Each identity word has a tag, and equals sign, and
some value. The tags are:
uuid= The value should be a 128 bit uuid in hexadecimal, with
punctuation interspersed if desired. This must match the
uuid stored in the superblock.
name= The value should be a simple textual name as was given to
mdadm when the array was created. This must match the
name stored in the superblock on a device for that device
to be included in the array. Not all superblock formats
The value is an integer which indicates the minor number
that was stored in the superblock when the array was
created. When an array is created as /dev/mdX, then the
minor number X is stored.
The value is a comma separated list of device names or
device name patterns. Only devices with names which match
one entry in the list will be used to assemble the array.
Note that the devices listed there must also be listed on
a DEVICE line.
level= The value is a RAID level. This is not normally used to
identify an array, but is supported so that the output of
mdadm --examine --scan
can be use directly in the configuration file.
The value is the number of devices in a complete active
array. As with level= this is mainly for compatibility
with the output of
mdadm --examine --scan.
The value is a number of spare devices to expect the array
to have. The sole use of this keyword and value is as
follows: mdadm --monitor will report an array if it is
found to have fewer than this number of spares when
--monitor starts or when --oneshot is used.
The value is a textual name for a group of arrays. All
arrays with the same spare-group name are considered to be
part of the same group. The significance of a group of
arrays is that mdadm will, when monitoring the arrays,
move a spare drive from one array in a group to another
array in that group if the first array had a failed or
missing drive but no spare.
auto= This option is rarely needed with mdadm-3.0, particularly
if use with the Linux kernel v2.6.28 or later. It tells
mdadm whether to use partitionable array or non-
partitionable arrays and, in the absence of udev, how many
partition devices to create. From 2.6.28 all md array
devices are partitionable, hence this option is not
The value of this option can be "yes" or "md" to indicate
that a traditional, non-partitionable md array should be
created, or "mdp", "part" or "partition" to indicate that
a partitionable md array (only available in linux 2.6 and
later) should be used. This later set can also have a
number appended to indicate how many partitions to create
device files for, e.g. auto=mdp5. The default is 4.
The option specifies a file in which a write-intent bitmap
should be found. When assembling the array, mdadm will
provide this file to the md driver as the bitmap file.
This has the same function as the --bitmap-file option to
Specify the metadata format that the array has. This is
mainly recognised for comparability with the output of
Specify that this array is a member array of some
container. The value given can be either a path name in
/dev, or a UUID of the container array.
Specify that this array is a member array of some
container. Each type of container has some way to
enumerate member arrays, often a simple sequence number.
The value identifies which member of a container the array
is. It will usually accompany a "container=" word.
The mailaddr line gives an E-mail address that alerts should
be sent to when mdadm is running in --monitor mode (and was
given the --scan option). There should only be one MAILADDR
line and it should have only one address. Any subsequent
addresses are silently ignored.
The mailfrom line (which can only be abbreviated to at least 5
characters) gives an address to appear in the "From" address
for alert mails. This can be useful if you want to explicitly
set a domain, as the default from address is "root" with no
domain. All words on this line are catenated with spaces to
form the address.
Note that this value cannot be set via the mdadm commandline.
It is only settable via the config file.
The program line gives the name of a program to be run when
mdadm --monitor detects potentially interesting events on any
of the arrays that it is monitoring. This program gets run
with two or three arguments, they being the Event, the md
device, and possibly the related component device.
There should only be one program line and it should be give
only one program.
CREATE The create line gives default values to be used when creating
arrays, new members of arrays, and device entries for arrays.
owner=group= These can give user/group ids or names to use instead of
system defaults (root/wheel or root/disk).
mode= An octal file mode such as 0660 can be given to override
the default of 0600.
auto= This corresponds to the --auto flag to mdadm. Give yes,
md, mdp, part — possibly followed by a number of
partitions — to indicate how missing device entries should
The name of the metadata format to use if none is
explicitly given. This can be useful to impose a system-
wide default of version-1 superblocks.
Normally when creating devices in /dev/md/ mdadm will
create a matching symlink from /dev/ with a name starting
md or md_. Give symlinks=no to suppress this symlink
Since Linux 2.6.29 it has been possible to create md
devices with a name like md_home rather than just a
number, like md3. mdadm will use the numeric alternative
by default as other tools that interact with md arrays may
expect only numbers. If names=yes is given in mdadm.conf
then mdadm will use a name when appropriate. If names=no
is given, then non-numeric md device names will not be
used even if the default changes in a future release of
bbl=no By default, mdadm will reserve space for a bad block list
(bbl) on all devices included in or added to any array
that supports them. Setting bbl=no will prevent this, so
newly added devices will not have a bad block log.
The homehost line gives a default value for the --homehost=
option to mdadm. There should normally be only one other word
on the line. It should either be a host name, or one of the
special words <system>, <none> and <ignore>. If <system> is
given, then the gethostname(2) systemcall is used to get the
host name. This is the default.
If <ignore> is given, then a flag is set so that when arrays
are being auto-assembled the checking of the recorded homehost
is disabled. If <ignore> is given it is also possible to give
an explicit name which will be used when creating arrays.
This is the only case when there can be more that one other
word on the HOMEHOST line. If there are other words, or other
HOMEHOST lines, they are silently ignored.
If <none> is given, then the default of using gethostname(2)
is over-ridden and no homehost name is assumed.
When arrays are created, this host name will be stored in the
metadata. When arrays are assembled using auto-assembly,
arrays which do not record the correct homehost name in their
metadata will be assembled using a "foreign" name. A
"foreign" name alway ends with a digit string preceded by an
underscore to differentiate it from any possible local name.
e.g. /dev/md/1_1 or /dev/md/home_0.
AUTO A list of names of metadata format can be given, each preceded
by a plus or minus sign. Also the word homehost is allowed as
is all preceded by plus or minus sign. all is usually last.
When mdadm is auto-assembling an array, either via --assemble
or --incremental and it finds metadata of a given type, it
checks that metadata type against those listed in this line.
The first match wins, where all matches anything. If a match
is found that was preceded by a plus sign, the auto assembly
is allowed. If the match was preceded by a minus sign, the
auto assembly is disallowed. If no match is found, the auto
assembly is allowed.
If the metadata indicates that the array was created for this
host, and the word homehost appears before any other match,
then the array is treated as a valid candidate for auto-
This can be used to disable all auto-assembly (so that only
arrays explicitly listed in mdadm.conf or on the command line
are assembled), or to disable assembly of certain metadata
types which might be handled by other software. It can also
be used to disable assembly of all foreign arrays - normally
such arrays are assembled but given a non-deterministic name
The known metadata types are 0.90, 1.x, ddf, imsm.
AUTO should be given at most once. Subsequent lines are
silently ignored. Thus an earlier config file in a config
directory will over-ride the setting in a later config file.
POLICY This is used to specify what automatic behavior is allowed on
devices newly appearing in the system and provides a way of
marking spares that can be moved to other arrays as well as
the migration domains. Domain can be defined through policy
line by specifying a domain name for a number of paths from
/dev/disk/by-path/. A device may belong to several domains.
The domain of an array is a union of domains of all devices in
that array. A spare can be automatically moved from one array
to another if the set of the destination array's domains
contains all the domains of the new disk or if both arrays
have the same spare-group.
To update hot plug configuration it is necessary to execute
mdadm --udev-rules command after changing the config file
Key words used in the POLICY line and supported values are:
any arbitrary string
0.9 1.x ddf or imsm
path= file glob matching anything from /dev/disk/by-pathtype= either disk or part.
include, re-add, spare, spare-same-slot, or force-spare
auto= yes, no, or homehost.
The action item determines the automatic behavior allowed for
devices matching the path and type in the same line. If a
device matches several lines with different actions then the
most permissive will apply. The ordering of policy lines is
irrelevant to the end result.
allows adding a disk to an array if metadata on that
disk matches that array
re-add will include the device in the array if it appears to
be a current member or a member that was recently
removed and the array has a write-intent-bitmap to
allow the re-add functionality.
spare as above and additionally: if the device is bare it can
become a spare if there is any array that it is a
candidate for based on domains and metadata.
as above and additionally if given slot was used by an
array that went degraded recently and the device
plugged in has no metadata then it will be
automatically added to that array (or it's container)
as above and the disk will become a spare in remaining
DEVICE /dev/hda1 /dev/hdb1
# /dev/md0 is known by its UUID.
ARRAY /dev/md0 UUID=3aaa0122:29827cfa:5331ad66:ca767371
# /dev/md1 contains all devices with a minor number of
# 1 in the superblock.
ARRAY /dev/md1 superminor=1
# /dev/md2 is made from precisely these two devices
ARRAY /dev/md2 devices=/dev/hda1,/dev/hdb1
# /dev/md4 and /dev/md5 are a spare-group and spares
# can be moved between them
ARRAY /dev/md4 uuid=b23f3c6d:aec43a9f:fd65db85:369432df
ARRAY /dev/md5 uuid=19464854:03f71b1b:e0df2edd:246cc977
# /dev/md/home is created if need to be a partitionable md array
# any spare device number is allocated.
ARRAY /dev/md/home UUID=9187a482:5dde19d9:eea3cc4a:d646ab8b
# The name of this array contains a space.
ARRAY /dev/md9 name='Data Storage'
POLICY domain=domain1 metadata=imsm path=pci-0000:00:1f.2-scsi-*
POLICY domain=domain1 metadata=imsm path=pci-0000:04:00.0-scsi-*
# One domain comprising of devices attached to specified paths is
# Bare device matching first path will be made an imsm spare on hot
# If more than one array is created on devices belonging to domain1
# one of them becomes degraded, then any imsm spare matching any path
# given domain name can be migrated.
CREATE group=system mode=0640 auto=part-8
AUTO +1.x homehost -all
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
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