bb636547b0
Document in Documentation/md.txt the files that now appear in sysfs, and make a couple of small refinements to exactly when 'level' and 'raid_disks' are empty, to make it match the documentation. Signed-off-by: Neil Brown <neilb@suse.de> Acked-by: Greg Kroah-Hartman <gregkh@suse.de> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
237 lines
8.6 KiB
Text
237 lines
8.6 KiB
Text
Tools that manage md devices can be found at
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http://www.<country>.kernel.org/pub/linux/utils/raid/....
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Boot time assembly of RAID arrays
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---------------------------------
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You can boot with your md device with the following kernel command
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lines:
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for old raid arrays without persistent superblocks:
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md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
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for raid arrays with persistent superblocks
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md=<md device no.>,dev0,dev1,...,devn
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or, to assemble a partitionable array:
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md=d<md device no.>,dev0,dev1,...,devn
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md device no. = the number of the md device ...
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0 means md0,
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1 md1,
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2 md2,
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3 md3,
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4 md4
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raid level = -1 linear mode
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0 striped mode
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other modes are only supported with persistent super blocks
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chunk size factor = (raid-0 and raid-1 only)
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Set the chunk size as 4k << n.
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fault level = totally ignored
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dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
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A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this:
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e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
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Boot time autodetection of RAID arrays
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--------------------------------------
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When md is compiled into the kernel (not as module), partitions of
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type 0xfd are scanned and automatically assembled into RAID arrays.
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This autodetection may be suppressed with the kernel parameter
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"raid=noautodetect". As of kernel 2.6.9, only drives with a type 0
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superblock can be autodetected and run at boot time.
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The kernel parameter "raid=partitionable" (or "raid=part") means
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that all auto-detected arrays are assembled as partitionable.
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Superblock formats
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------------------
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The md driver can support a variety of different superblock formats.
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Currently, it supports superblock formats "0.90.0" and the "md-1" format
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introduced in the 2.5 development series.
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The kernel will autodetect which format superblock is being used.
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Superblock format '0' is treated differently to others for legacy
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reasons - it is the original superblock format.
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General Rules - apply for all superblock formats
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------------------------------------------------
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An array is 'created' by writing appropriate superblocks to all
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devices.
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It is 'assembled' by associating each of these devices with an
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particular md virtual device. Once it is completely assembled, it can
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be accessed.
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An array should be created by a user-space tool. This will write
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superblocks to all devices. It will usually mark the array as
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'unclean', or with some devices missing so that the kernel md driver
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can create appropriate redundancy (copying in raid1, parity
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calculation in raid4/5).
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When an array is assembled, it is first initialized with the
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SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor
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version number. The major version number selects which superblock
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format is to be used. The minor number might be used to tune handling
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of the format, such as suggesting where on each device to look for the
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superblock.
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Then each device is added using the ADD_NEW_DISK ioctl. This
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provides, in particular, a major and minor number identifying the
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device to add.
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The array is started with the RUN_ARRAY ioctl.
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Once started, new devices can be added. They should have an
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appropriate superblock written to them, and then passed be in with
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ADD_NEW_DISK.
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Devices that have failed or are not yet active can be detached from an
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array using HOT_REMOVE_DISK.
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Specific Rules that apply to format-0 super block arrays, and
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arrays with no superblock (non-persistent).
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-------------------------------------------------------------
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An array can be 'created' by describing the array (level, chunksize
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etc) in a SET_ARRAY_INFO ioctl. This must has major_version==0 and
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raid_disks != 0.
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Then uninitialized devices can be added with ADD_NEW_DISK. The
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structure passed to ADD_NEW_DISK must specify the state of the device
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and it's role in the array.
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Once started with RUN_ARRAY, uninitialized spares can be added with
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HOT_ADD_DISK.
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MD devices in sysfs
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-------------------
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md devices appear in sysfs (/sys) as regular block devices,
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e.g.
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/sys/block/md0
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Each 'md' device will contain a subdirectory called 'md' which
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contains further md-specific information about the device.
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All md devices contain:
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level
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a text file indicating the 'raid level'. This may be a standard
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numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
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other name such as "linear" or "multipath".
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If no raid level has been set yet (array is still being
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assembled), this file will be empty.
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raid_disks
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a text file with a simple number indicating the number of devices
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in a fully functional array. If this is not yet known, the file
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will be empty. If an array is being resized (not currently
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possible) this will contain the larger of the old and new sizes.
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As component devices are added to an md array, they appear in the 'md'
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directory as new directories named
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dev-XXX
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where XXX is a name that the kernel knows for the device, e.g. hdb1.
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Each directory contains:
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block
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a symlink to the block device in /sys/block, e.g.
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/sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
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super
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A file containing an image of the superblock read from, or
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written to, that device.
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state
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A file recording the current state of the device in the array
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which can be a comma separated list of
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faulty - device has been kicked from active use due to
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a detected fault
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in_sync - device is a fully in-sync member of the array
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spare - device is working, but not a full member.
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This includes spares that are in the process
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of being recoverred to
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This list make grow in future.
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An active md device will also contain and entry for each active device
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in the array. These are named
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rdNN
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where 'NN' is the possition in the array, starting from 0.
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So for a 3 drive array there will be rd0, rd1, rd2.
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These are symbolic links to the appropriate 'dev-XXX' entry.
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Thus, for example,
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cat /sys/block/md*/md/rd*/state
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will show 'in_sync' on every line.
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Active md devices for levels that support data redundancy (1,4,5,6)
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also have
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sync_action
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a text file that can be used to monitor and control the rebuild
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process. It contains one word which can be one of:
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resync - redundancy is being recalculated after unclean
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shutdown or creation
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recover - a hot spare is being built to replace a
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failed/missing device
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idle - nothing is happening
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check - A full check of redundancy was requested and is
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happening. This reads all block and checks
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them. A repair may also happen for some raid
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levels.
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repair - A full check and repair is happening. This is
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similar to 'resync', but was requested by the
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user, and the write-intent bitmap is NOT used to
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optimise the process.
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This file is writable, and each of the strings that could be
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read are meaningful for writing.
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'idle' will stop an active resync/recovery etc. There is no
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guarantee that another resync/recovery may not be automatically
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started again, though some event will be needed to trigger
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this.
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'resync' or 'recovery' can be used to restart the
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corresponding operation if it was stopped with 'idle'.
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'check' and 'repair' will start the appropriate process
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providing the current state is 'idle'.
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mismatch_count
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When performing 'check' and 'repair', and possibly when
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performing 'resync', md will count the number of errors that are
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found. The count in 'mismatch_cnt' is the number of sectors
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that were re-written, or (for 'check') would have been
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re-written. As most raid levels work in units of pages rather
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than sectors, this my be larger than the number of actual errors
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by a factor of the number of sectors in a page.
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Each active md device may also have attributes specific to the
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personality module that manages it.
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These are specific to the implementation of the module and could
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change substantially if the implementation changes.
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These currently include
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stripe_cache_size (currently raid5 only)
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number of entries in the stripe cache. This is writable, but
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there are upper and lower limits (32768, 16). Default is 128.
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strip_cache_active (currently raid5 only)
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number of active entries in the stripe cache
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