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.
SYNTAX
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.
super-minor=
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.
devices=
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.
num-devices=
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.
spares=
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.
spare-group=
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.
bitmap=
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.
metadata=
Specify the metadata format that the array has. This
is mainly recognised for comparability with the output
of mdadm -Es.
container=
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.
member=
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.
MAILADDR
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.
MAILFROM
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.
PROGRAM
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:
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 be created.
metadata=
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.
symlinks=no
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.
names=yes
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.
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.
HOMEHOST
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-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 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:
domain=
any arbitrary string
metadata=
0.9 1.x ddf or imsm
path= file glob matching anything from /dev/disk/by-path
type= either disk or part.
action=
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.
include
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.
spare-same-slot
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)
force-spare
as above and the disk will become a spare in remaining
cases
PART-POLICY
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
relates.
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.
MONITORDELAY
The monitordelay line gives a delay in seconds mdadm shall
wait before pooling md arrays when mdadm is running in
--monitor mode. -d/--delay command line argument takes
precedence over the config file
