mdadm.conf(5) — Linux manual page


MDADM.CONF(5)                File Formats Manual               MDADM.CONF(5)

NAME         top

       mdadm.conf - configuration for management of Software RAID with mdadm

SYNOPSIS         top


DESCRIPTION         top

       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.

              For example:

              DEVICE /dev/hda* /dev/hdc*
              DEV    /dev/sd*
              DEVICE /dev/disk/by-path/pci*
              DEVICE partitions

       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
                  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 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
                  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 "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.
              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

                  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
              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
              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

              Keywords 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 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

              This is similar to POLICY and accepts the same keyword
              assignments.  It allows a consistent set of policies to
              applied to each of the partitions of a device.

              A PART-POLICY line should set type=disk and identify the path
              to one or more disk devices.  Each partition on these disks
              will be treated according to the action= setting  from this
              line.  If a domain is set in the line, then the domain
              associated with each patition will be based on the domain, but
              with "-partN" appended, when N is the partition number for the
              partition that was found.

       SYSFS  The SYSFS line lists custom values of MD device's sysfs
              attributes which will be stored in sysfs after the array is
              assembled. Multiple lines are allowed and each line has to
              contain the uuid or the name of the device to which it

           uuid=  hexadecimal identifier of MD device. This has to match the
                  uuid stored in the superblock.

           name=  name of the MD device as was given to mdadm when the array
                  was created. It will be ignored if uuid is not empty.

EXAMPLE         top

              DEVICE /dev/sd[bcdjkl]1
              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
              # 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
              POLICY domain=domain1 metadata=imsm
              # One domain comprising of devices attached to specified paths
              is defined.
              # Bare device matching first path will be made an imsm spare
              on hot plug.
              # 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 for
              # given domain name can be migrated.
              MAILADDR root@mydomain.tld
              PROGRAM /usr/sbin/handle-mdadm-events
              CREATE group=system mode=0640 auto=part-8
              HOMEHOST <system>
              AUTO +1.x homehost -all
              SYSFS name=/dev/md/raid5 group_thread_cnt=4
              SYSFS uuid=bead5eb6:31c17a27:da120ba2:7dfda40d
              group_thread_cnt=4 sync_speed_max=1000000

SEE ALSO         top

       mdadm(8), md(4).

COLOPHON         top

       This page is part of the mdadm (Tool for managing md arrays in Linux)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for
       this manual page, send it to  This page
       was obtained from the project's upstream Git repository
       ⟨⟩ on
       2020-11-01.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-10-14.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to


Pages that refer to this page: mdadm(8)raid6check(8)