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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