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Tools that manage md devices can be found at
Boot time assembly of RAID arrays
You can boot with your md device with the following kernel command
for old raid arrays without persistent superblocks:
md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
for raid arrays with persistent superblocks
md=<md device no.>,dev0,dev1,...,devn
or, to assemble a partitionable array:
md=d<md device no.>,dev0,dev1,...,devn
md device no. = the number of the md device ...
0 means md0,
1 md1,
2 md2,
3 md3,
4 md4
raid level = -1 linear mode
0 striped mode
other modes are only supported with persistent super blocks
chunk size factor = (raid-0 and raid-1 only)
Set the chunk size as 4k << n.
fault level = totally ignored
dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this:
e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
Boot time autodetection of RAID arrays
When md is compiled into the kernel (not as module), partitions of
type 0xfd are scanned and automatically assembled into RAID arrays.
This autodetection may be suppressed with the kernel parameter
"raid=noautodetect". As of kernel 2.6.9, only drives with a type 0
superblock can be autodetected and run at boot time.
The kernel parameter "raid=partitionable" (or "raid=part") means
that all auto-detected arrays are assembled as partitionable.
Superblock formats
The md driver can support a variety of different superblock formats.
Currently, it supports superblock formats "0.90.0" and the "md-1" format
introduced in the 2.5 development series.
The kernel will autodetect which format superblock is being used.
Superblock format '0' is treated differently to others for legacy
reasons - it is the original superblock format.
General Rules - apply for all superblock formats
An array is 'created' by writing appropriate superblocks to all
It is 'assembled' by associating each of these devices with an
particular md virtual device. Once it is completely assembled, it can
be accessed.
An array should be created by a user-space tool. This will write
superblocks to all devices. It will usually mark the array as
'unclean', or with some devices missing so that the kernel md driver
can create appropriate redundancy (copying in raid1, parity
calculation in raid4/5).
When an array is assembled, it is first initialized with the
SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor
version number. The major version number selects which superblock
format is to be used. The minor number might be used to tune handling
of the format, such as suggesting where on each device to look for the
Then each device is added using the ADD_NEW_DISK ioctl. This
provides, in particular, a major and minor number identifying the
device to add.
The array is started with the RUN_ARRAY ioctl.
Once started, new devices can be added. They should have an
appropriate superblock written to them, and then passed be in with
Devices that have failed or are not yet active can be detached from an
array using HOT_REMOVE_DISK.
Specific Rules that apply to format-0 super block arrays, and
arrays with no superblock (non-persistent).
An array can be 'created' by describing the array (level, chunksize
etc) in a SET_ARRAY_INFO ioctl. This must has major_version==0 and
raid_disks != 0.
Then uninitialized devices can be added with ADD_NEW_DISK. The
structure passed to ADD_NEW_DISK must specify the state of the device
and it's role in the array.
Once started with RUN_ARRAY, uninitialized spares can be added with
MD devices in sysfs
md devices appear in sysfs (/sys) as regular block devices,
Each 'md' device will contain a subdirectory called 'md' which
contains further md-specific information about the device.
All md devices contain:
a text file indicating the 'raid level'. This may be a standard
numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
other name such as "linear" or "multipath".
If no raid level has been set yet (array is still being
assembled), this file will be empty.
a text file with a simple number indicating the number of devices
in a fully functional array. If this is not yet known, the file
will be empty. If an array is being resized (not currently
possible) this will contain the larger of the old and new sizes.
As component devices are added to an md array, they appear in the 'md'
directory as new directories named
where XXX is a name that the kernel knows for the device, e.g. hdb1.
Each directory contains:
a symlink to the block device in /sys/block, e.g.
/sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
A file containing an image of the superblock read from, or
written to, that device.
A file recording the current state of the device in the array
which can be a comma separated list of
faulty - device has been kicked from active use due to
a detected fault
in_sync - device is a fully in-sync member of the array
spare - device is working, but not a full member.
This includes spares that are in the process
of being recoverred to
This list make grow in future.
An active md device will also contain and entry for each active device
in the array. These are named
where 'NN' is the possition in the array, starting from 0.
So for a 3 drive array there will be rd0, rd1, rd2.
These are symbolic links to the appropriate 'dev-XXX' entry.
Thus, for example,
cat /sys/block/md*/md/rd*/state
will show 'in_sync' on every line.
Active md devices for levels that support data redundancy (1,4,5,6)
also have
a text file that can be used to monitor and control the rebuild
process. It contains one word which can be one of:
resync - redundancy is being recalculated after unclean
shutdown or creation
recover - a hot spare is being built to replace a
failed/missing device
idle - nothing is happening
check - A full check of redundancy was requested and is
happening. This reads all block and checks
them. A repair may also happen for some raid
repair - A full check and repair is happening. This is
similar to 'resync', but was requested by the
user, and the write-intent bitmap is NOT used to
optimise the process.
This file is writable, and each of the strings that could be
read are meaningful for writing.
'idle' will stop an active resync/recovery etc. There is no
guarantee that another resync/recovery may not be automatically
started again, though some event will be needed to trigger
'resync' or 'recovery' can be used to restart the
corresponding operation if it was stopped with 'idle'.
'check' and 'repair' will start the appropriate process
providing the current state is 'idle'.
When performing 'check' and 'repair', and possibly when
performing 'resync', md will count the number of errors that are
found. The count in 'mismatch_cnt' is the number of sectors
that were re-written, or (for 'check') would have been
re-written. As most raid levels work in units of pages rather
than sectors, this my be larger than the number of actual errors
by a factor of the number of sectors in a page.
Each active md device may also have attributes specific to the
personality module that manages it.
These are specific to the implementation of the module and could
change substantially if the implementation changes.
These currently include
stripe_cache_size (currently raid5 only)
number of entries in the stripe cache. This is writable, but
there are upper and lower limits (32768, 16). Default is 128.
strip_cache_active (currently raid5 only)
number of active entries in the stripe cache