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Diffstat (limited to 'Documentation/md.txt')
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diff --git a/Documentation/md.txt b/Documentation/md.txt new file mode 100644 index 000000000000..e2b536992a27 --- /dev/null +++ b/Documentation/md.txt @@ -0,0 +1,118 @@ +Tools that manage md devices can be found at + http://www.<country>.kernel.org/pub/linux/utils/raid/.... + + +Boot time assembly of RAID arrays +--------------------------------- + +You can boot with your md device with the following kernel command +lines: + +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 +devices. + +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 +superblock. + +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 +ADD_NEW_DISK. + +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 +HOT_ADD_DISK. |