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 (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 the word partitions.
This 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 num-
If no DEVICE line is present, then "DEVICE partitions" is
DEVICE /dev/hda* /dev/hdc*
ARRAY The ARRAY lines identify actual arrays. The second word on the
line should be the name of the device where the array is nor-
mally assembled, such as /dev/md1. Subsequent words identify
the array, or identify the array as a member of a group. If mul-
tiple 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 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. mdadm --monitor will report an array if it is
found to have fewer than this number of spares when --moni-
tor 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 declares to mdadm that it should try to create
the device file of the array if it doesn't already exist, or
exists but with the wrong device number.
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
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
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.
The homehost line gives a default value for the --homehost=
option to mdadm. There should be exactly one other word on the
line. It should either exactly <<system>> or a host name. If
<<system>> is given, then the gethostname(2) systemcall is used to
get the host name. When arrays are created, this host name will
be stored in the metadata. When arrays are assembled using
auto-assembly, only arrays with this host name stored in the
metadata will be considered.
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