mdadm.conf - configuration for management of Software RAID with mdadm
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 (sepa‐
rated 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 contain‐
ers" 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 com‐
ponent 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 support
The value is an integer which indicates the minor number
that was stored in the superblock when the array was cre‐
ated. 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
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 fol‐
lows: 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 needed.
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 num‐
ber 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 pro‐
vide 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 mdadm
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 identi‐
fies which member of a container the array is. It will usu‐
ally 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
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
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
CREATE The create line gives default values to be used when creating
arrays and device entries for arrays. These include:
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 be created.
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 creation.
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 mdadm.
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 dif‐
ferentiate it from any possible local name. e.g. /dev/md/1_1 or
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-assembly.
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 in /dev/md/.
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 ppcontains
all the domains of the new disk or if both arrays have the same
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-path
type= 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
spare as above and additionally: if the device is bare it can
become a spare if there is any array that it is a candi‐
date 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 and
# 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
SEE ALSOmdadm(8), md(4).