MDADM.CONF(5) File Formats Manual MDADM.CONF(5)
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.
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 of 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
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
Subsequent words identify the array, or identify the array as a member of a group. If multiple identities are given, then a compo-
nent device must match ALL identities to be considered a match. Each identity word has a tag, and equals sign, and some value. The
uuid= The value should be a 128 bit uuid in hexadecimal, with punctuation interspersed if desired. This must match the uuid stored in
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 names.
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
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
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 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 --assemble.
Specify the metadata format that the array has. This is mainly recognised for comparability with the output of mdadm -Es.
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
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.
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 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 ver-
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.
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> and <ignore>. If <system> is given, then the gethost-
name(2) systemcall is used to get the host name.
If <ignore> is given, then a flag is set so that when arrays are being auto-assembled the checking of the recorded homehost is dis-
abled. 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.
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-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 assem-
bled), 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.
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
CREATE group=system mode=0640 auto=part-8
AUTO +1.x homehost -all