path: root/mdadm.8

.\" -*- nroff -*-
.TH mdadm 8
.SH NAME
mdadm \- manage MD devices
.I aka
Linux Software Raid.

.SH SYNOPSIS

.BI mdadm " [mode] <raiddevice> [options] <subdevices>"

.SH DESCRIPTION 
RAID devices are virtual devices created from two or more
real block devices. This allows multiple devices (typically disk
drives or partitions there-of) to be combined into a single device to
hold (for example) a single filesystem.
Some RAID levels included redundancy and so can survive some degree of
device failure.

Linux Software RAID devices are implemented through the md (Multiple Devices) device driver.

Currently, Linux supports
.B LINEAR
md devices,
.B RAID0
(striping),
.B RAID1
(mirroring),
.B RAID4
and
.B RAID5.

Recent kernels (2002) also support a mode known as
.BR MULTIPATH .
.B mdadm
does not support MULTIPATH as yet.

.B mdadm
is a program that can be used to create, manage, and monitor
MD devices.  As
such it provides a similar set of functionality to the
.B raidtools
packages.
The key differences between
.B mdadm
and
.B raidtools
are:
.IP \(bu 4
.B mdadm
is a single program and not a collection of programs.
.IP \(bu 4
.B mdadm
can perform (almost) all of its functions without having a
configuration file.  Also mdadm helps with management of the configuration
file.
.IP \(bu 4
.B mdadm
can provide information about your arrays (through Detail and Examine)
that
.B  raidtools
cannot.
.IP \(bu 4
.B raidtools
can manage MULTIPATH devices which
.B mdadm
cannot yet manage.

.SH MODES
mdadm has 7 major modes of operation:
.TP
.B Assemble
Assemble the parts of a previously created
array into an active array. Components can be explicitly given
or can be searched for. 
.B mdadm
checks that the components
do form a bona fide array, and can, on request, fiddle superblock
information so as to assemble a faulty array.

.TP
.B Build
Build a legacy array without per-device superblocks.

.TP
.B Create
Create a new array with per-device superblocks.
'''It can progress
'''in several step create-add-add-run or it can all happen with one command.

.TP
.B Detail
Display the details of a given md device.  Details include the RAID
level, the number of devices, which ones are faulty (if any), and the
array UUID.

.TP
.B Examine
Examine a device to see if it is part of an md array, and print out
the details of that array.
This mode can also be used to examine a large number of devices and to
print out a summary of the arrays found in a format suitable for the
.B mdadm.conf
configuration file.

.TP
.B "Follow or Monitor"
Monitor one or more md devices and act on any state changes.

.TP
.B Manage
This is for odd bits an pieces like hotadd, hotremove, setfaulty, stop,
readonly, readwrite.
'''If an array is only partially setup by the
'''Create or Assemble commands, subsequent Manage commands can finish the
'''job.

.SH OPTIONS

Available options are:

.TP
.BR -A ", " --assemble
Assemble an existing array.

.TP
.BR -B ", " --build
Build a legacy array without superblocks.

.TP
.BR -C ", " --create
Create a new array.

.TP
.BR -D ", " --detail
Print detail of one or more md devices.

.TP
.BR -E ", " --examine
Print content of md superblock on device(s).

.TP
.BR -F ", " --follow ", " --monitor
Select
.B Monitor
mode.

.TP
.BR -h ", " --help
Display help message or, after above option, mode specific help message.

.TP
.BR -V ", " --version
Print version information for mdadm.

.TP
.BR -v ", " --verbose
Be more verbose about what is happening.

.TP
.BR -b ", " --brief
Be less verbose.  This is used with
.B --detail
and
.BR --examine .

.SH For create or build:

.TP
.BR -c ", " --chunk=
Specify chunk size of kibibytes.  The default is 64.

.TP
.BR --rounding=
Specify rounding factor for linear array (==chunk size)

.TP
.BR -l ", " --level=
Set raid level.  Options are: linear, raid0, 0, stripe, raid1, 1, mirror, raid5, 4,
raid5, 5.  Obviously some of these are synonymous.
Only the first 4 are valid when Building.

.TP
.BR -p ", " --parity=
Set raid5 parity algorithm. Options are:
{left,right}-{,a}symmetric, la, ra, ls, rs.  The default is left-symmetric.

.TP
.BR --layout=
same as --parity

.TP
.BR -n ", " --raid-disks=
number of active devices in array.

.TP
.BR -x ", " --spare-disks=
number of spare (eXtra) disks in initial array.  Spares can be added
and removed later.

.TP
.BR -z ", " --size=
Amount (in Kibibytes) of space to use from each drive in RAID1/4/5.
This must be a multiple of the chunk size, and must leave about 128Kb
of space at the end of the drive for the RAID superblock.
If this is not specified
(as it normally is not) the smallest drive (or partition) sets the
size, though if there is a variance among the drives of greater than 1%, a warning is
issued.

.SH For assemble:

.TP
.BR -u ", " --uuid=
uuid of array to assemble. Devices which don't have this uuid are
excluded

.TP
.BR -m ", " --super-minor=
Minor number of device that array was created for.  Devices which
don't have this minor number are excluded.  If you create an array as
/dev/md1, then all superblock will contain the minor number 1, even if
the array is later assembled as /dev/md2.

.TP
.BR -c ", " --config=
config file.  Default is
.BR /etc/mdadm.conf .

.TP
.BR -s ", " --scan
scan config file for missing information

.TP
.BR -f ", " --force
Assemble the array even if some superblocks appear out-of-date

.TP
.BR -R ", " --run
Attempt to start the array even if fewer drives were given than are
needed for a full array. Normally if not all drives are found and
.B --scan
is not used, then the array will be assembled but not started.
With
.B --run
an attempt will be made to start it anyway.

.SH General management

.TP
.BR -a ", " --add
'''add, or
hotadd listed devices.

.TP
.BR -r ", " --remove
remove listed devices.  The must not be active.  i.e. they should
be failed or spare devices.

.TP
.BR -f ", " --fail
mark listed devices as faulty.

.TP
.BR --set-faulty
same as --fail.

.TP
.BR -R ", " --run
start a partially built array.

.TP
.BR -S ", " --stop
deactivate array, releasing all resources.

.TP
.BR -o ", " --readonly
mark array as readonly.

.TP
.BR -w ", " --readwrite
mark array as readwrite.


.SH ASSEMBLY MODE

.HP 12
Usage:
.B mdadm --assemble
.I device options...
.HP 12
Usage:
.B mdadm --assemble --scan
.I  options...

.PP
This usage assembles one or more raid arrays from pre-existing components.
For each array, mdadm needs to know the md device, the identity of the
array, and a number of sub devices. These can be found in a number of ways.

The md device is either given before 
.B --scan
or is found from the config file. In the latter case, multiple md devices
can be started with a single mdadm command.

The identity can be given with the 
.B --uuid
option, with the
.B --super-minor
option, can be found in in the config file, or will be taken from the
super block on the first subdevice listed on the command line.

Devices can be given on the 
.B --assemble
command line or from the config file. Only devices which have an md
superblock which contains the right identity will be considered for any device.

The config file is only used if explicitly named with 
.B --config
or requested with 
.B --scan. 
In the later case,
.B /etc/mdadm.conf
is used.

If 
.B --scan
is not given, then the config file will only be used to find the
identity of md arrays.

Normally the array will be started after it is assembled.  However is
.B --scan
is not given and insufficient drives were lists to start a complete
(non-degraded) array, then the array is not started (to guard against
usage errors).  To insist that the array be started in this case (as
may work for RAID1 or RAID5), give the
.B --run
flag.


.SH BUILD MODE

.HP 12
Usage:
.B mdadm --build
.I device
.BI --chunk= X
.BI --level= Y
.BI --raid-disks= Z
.I devices

.PP
This usage is similar to 
.BR --create .
The difference is that it creates a legacy array without a superblock. With
these arrays there is no difference between initially creating the array and
subsequently assembling the array, except that hopefully there is useful
data there in the second case.

The level may only be 0, raid0, or linear. All devices must be listed
and the array will be started once complete.

.SH CREATE MODE

.HP 12
Usage:
.B mdadm --create
.I device
.BI --chunk= X
.BI --level= Y
.br
.BI --raid-disks= Z
.I  devices

.PP
This usage will initialise a new md array, associate some devices with
it, and activate the array.

As devices are added, they are checked to see if they contain raid
superblocks or filesystems. They are also check to see if the variance in
device size exceeds 1%.

If any discrepancy is found, the array will not automatically be run, though
the presence of a 
.B --run
can override this caution.

'''If the 
'''.B --size
'''option is given, it is not necessary to list any subdevices in this command.
'''They can be added later, before a
'''.B --run. 
'''If no 
'''.B --size
'''is given, the apparent size of the smallest drive given is used.

The General Management options that are valid with --create are:
.TP
.B --run
insist of running the array even if some devices look like they might
be in use.

.TP
.B --readonly
start the array readonly - not supported yet.

.SH DETAIL MODE
.HP 12
Usage:
.B mdadm --detail
.RB [ --brief ]
.I device ...
.PP

This usage sill print out the details of the given array including a
list of component devices.  To determine names for the devices,
.B mdadm
searches
.B /dev
for device files with the right major and minor numbers.

With
.B --brief
.B mdadm
prints a single line that identifies the level, number of disks, and
UUID of the array.  This line is suitable for inclusion in
.BR /etc/mdadm.conf .

.SH EXAMINE MODE
.HP 12
Usage:
.B mdadm --examine
.RB [ --scan ]
.RB [ --brief ]
.I device ...
.PP
This usage will examine some block devices to see if that have a valid
RAID superblock on them.  The information in each valid raid
superblock will be printed.

If
.B --scan
is used, the no devices should be listed, and the complete set of
devices identified in the configuration file are checked.
.B --scan
implies
.B --brief
but this implication can be countered by specifying
.BR --verbose .

With
.B --brief
.B mdadm
will output an config file entry of each distinct array that was
found.  This entry will list the UUID, the raid level, and a list of
the individual devices on which a superblock for that array was found.
This output will by syntactically suitable for inclusion in the
configuration file, but should
.B NOT
be used blindly.  Often the array description that you want in the
configuration file is much less specific than that given by
.BR "mdadm -Bs" .
For example, you normally do not want to list the devices,
particularly if they are SCSI devices.

'''.SH BUGS
'''no known bugs.

.SH FILES

.SS /proc/mdstat

If you're using the 
.B /proc 
filesystem,
.B /proc/mdstat
gives you informations about md devices status.
This file is not currently used by
.BR mdadm .

.SS /etc/mdadm.conf

The config file lists which devices may be scanned to see if
they contain MD super block, and gives identifying information
(e.g. UUID) about known MD arrays.  See
.BR mdadm.conf (5)
for more details.


.SH TODO

Finish and document Follow mode.

.SH SEE ALSO
For information on the various levels of
RAID, check out:

.IP
.UR   http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/
http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/
.UE
.PP
for new releases of the RAID driver check out:

.IP
.UR  ftp://ftp.kernel.org/pub/linux/kernel/people/mingo/raid-patches
ftp://ftp.kernel.org/pub/linux/kernel/people/mingo/raid-patches
.UE
.PP
or
.IP
.UR http://www.cse.unsw.edu.au/~neilb/patches/linux-stable/
http://www.cse.unsw.edu.au/~neilb/patches/linux-stable/
.URk
.PP
.BR mdadm.conf (5),
.BR md (4).
.PP
.IR raidtab (5),
.IR raid0run (8),
.IR raidstop (8),
.IR mkraid (8)