459 lines
18 KiB
Plaintext
459 lines
18 KiB
Plaintext
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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Boot time assembly of degraded/dirty arrays
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-------------------------------------------
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If a raid5 or raid6 array is both dirty and degraded, it could have
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undetectable data corruption. This is because the fact that it is
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'dirty' means that the parity cannot be trusted, and the fact that it
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is degraded means that some datablocks are missing and cannot reliably
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be reconstructed (due to no parity).
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For this reason, md will normally refuse to start such an array. This
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requires the sysadmin to take action to explicitly start the array
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despite possible corruption. This is normally done with
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mdadm --assemble --force ....
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This option is not really available if the array has the root
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filesystem on it. In order to support this booting from such an
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array, md supports a module parameter "start_dirty_degraded" which,
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when set to 1, bypassed the checks and will allows dirty degraded
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arrays to be started.
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So, to boot with a root filesystem of a dirty degraded raid[56], use
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md-mod.start_dirty_degraded=1
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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'. e.g. raid0, raid1,
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raid5, linear, multipath, faulty.
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If no raid level has been set yet (array is still being
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assembled), the value will reflect whatever has been written
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to it, which may be a name like the above, or may be a number
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such as '0', '5', etc.
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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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Some raid level (RAID1) allow this value to be set while the
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array is active. This will reconfigure the array. Otherwise
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it can only be set while assembling an array.
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chunk_size
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This is the size if bytes for 'chunks' and is only relevant to
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raid levels that involve striping (1,4,5,6,10). The address space
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of the array is conceptually divided into chunks and consecutive
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chunks are striped onto neighbouring devices.
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The size should be at least PAGE_SIZE (4k) and should be a power
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of 2. This can only be set while assembling an array
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layout
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The "layout" for the array for the particular level. This is
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simply a number that is interpretted differently by different
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levels. It can be written while assembling an array.
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reshape_position
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This is either "none" or a sector number within the devices of
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the array where "reshape" is up to. If this is set, the three
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attributes mentioned above (raid_disks, chunk_size, layout) can
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potentially have 2 values, an old and a new value. If these
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values differ, reading the attribute returns
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new (old)
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and writing will effect the 'new' value, leaving the 'old'
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unchanged.
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component_size
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For arrays with data redundancy (i.e. not raid0, linear, faulty,
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multipath), all components must be the same size - or at least
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there must a size that they all provide space for. This is a key
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part or the geometry of the array. It is measured in sectors
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and can be read from here. Writing to this value may resize
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the array if the personality supports it (raid1, raid5, raid6),
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and if the component drives are large enough.
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metadata_version
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This indicates the format that is being used to record metadata
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about the array. It can be 0.90 (traditional format), 1.0, 1.1,
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1.2 (newer format in varying locations) or "none" indicating that
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the kernel isn't managing metadata at all.
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resync_start
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The point at which resync should start. If no resync is needed,
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this will be a very large number. At array creation it will
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default to 0, though starting the array as 'clean' will
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set it much larger.
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new_dev
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This file can be written but not read. The value written should
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be a block device number as major:minor. e.g. 8:0
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This will cause that device to be attached to the array, if it is
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available. It will then appear at md/dev-XXX (depending on the
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name of the device) and further configuration is then possible.
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safe_mode_delay
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When an md array has seen no write requests for a certain period
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of time, it will be marked as 'clean'. When another write
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request arrives, the array is marked as 'dirty' before the write
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commences. This is known as 'safe_mode'.
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The 'certain period' is controlled by this file which stores the
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period as a number of seconds. The default is 200msec (0.200).
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Writing a value of 0 disables safemode.
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array_state
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This file contains a single word which describes the current
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state of the array. In many cases, the state can be set by
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writing the word for the desired state, however some states
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cannot be explicitly set, and some transitions are not allowed.
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clear
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No devices, no size, no level
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Writing is equivalent to STOP_ARRAY ioctl
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inactive
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May have some settings, but array is not active
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all IO results in error
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When written, doesn't tear down array, but just stops it
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suspended (not supported yet)
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All IO requests will block. The array can be reconfigured.
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Writing this, if accepted, will block until array is quiessent
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readonly
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no resync can happen. no superblocks get written.
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write requests fail
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read-auto
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like readonly, but behaves like 'clean' on a write request.
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clean - no pending writes, but otherwise active.
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When written to inactive array, starts without resync
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If a write request arrives then
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if metadata is known, mark 'dirty' and switch to 'active'.
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if not known, block and switch to write-pending
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If written to an active array that has pending writes, then fails.
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active
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fully active: IO and resync can be happening.
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When written to inactive array, starts with resync
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write-pending
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clean, but writes are blocked waiting for 'active' to be written.
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active-idle
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like active, but no writes have been seen for a while (safe_mode_delay).
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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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writemostly - device will only be subject to read
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requests if there are no other options.
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This applies only to raid1 arrays.
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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 recovered to
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This list may grow in future.
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This can be written to.
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Writing "faulty" simulates a failure on the device.
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Writing "remove" removes the device from the array.
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Writing "writemostly" sets the writemostly flag.
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Writing "-writemostly" clears the writemostly flag.
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errors
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An approximate count of read errors that have been detected on
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this device but have not caused the device to be evicted from
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the array (either because they were corrected or because they
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happened while the array was read-only). When using version-1
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metadata, this value persists across restarts of the array.
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This value can be written while assembling an array thus
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providing an ongoing count for arrays with metadata managed by
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userspace.
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slot
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This gives the role that the device has in the array. It will
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either be 'none' if the device is not active in the array
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(i.e. is a spare or has failed) or an integer less than the
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'raid_disks' number for the array indicating which position
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it currently fills. This can only be set while assembling an
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array. A device for which this is set is assumed to be working.
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offset
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This gives the location in the device (in sectors from the
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start) where data from the array will be stored. Any part of
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the device before this offset us not touched, unless it is
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used for storing metadata (Formats 1.1 and 1.2).
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size
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The amount of the device, after the offset, that can be used
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for storage of data. This will normally be the same as the
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component_size. This can be written while assembling an
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array. If a value less than the current component_size is
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written, component_size will be reduced to this value.
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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 position 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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bitmap_set_bits
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If the array has a write-intent bitmap, then writing to this
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attribute can set bits in the bitmap, indicating that a resync
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would need to check the corresponding blocks. Either individual
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numbers or start-end pairs can be written. Multiple numbers
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can be separated by a space.
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Note that the numbers are 'bit' numbers, not 'block' numbers.
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They should be scaled by the bitmap_chunksize.
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sync_speed_min
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sync_speed_max
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This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
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however they only apply to the particular array.
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If no value has been written to these, of if the word 'system'
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is written, then the system-wide value is used. If a value,
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in kibibytes-per-second is written, then it is used.
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When the files are read, they show the currently active value
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followed by "(local)" or "(system)" depending on whether it is
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a locally set or system-wide value.
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sync_completed
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This shows the number of sectors that have been completed of
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whatever the current sync_action is, followed by the number of
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sectors in total that could need to be processed. The two
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numbers are separated by a '/' thus effectively showing one
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value, a fraction of the process that is complete.
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A 'select' on this attribute will return when resync completes,
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when it reaches the current sync_max (below) and possibly at
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other times.
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sync_max
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This is a number of sectors at which point a resync/recovery
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process will pause. When a resync is active, the value can
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only ever be increased, never decreased. The value of 'max'
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effectively disables the limit.
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sync_speed
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This shows the current actual speed, in K/sec, of the current
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sync_action. It is averaged over the last 30 seconds.
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suspend_lo
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suspend_hi
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The two values, given as numbers of sectors, indicate a range
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within the array where IO will be blocked. This is currently
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only supported for raid4/5/6.
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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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preread_bypass_threshold (currently raid5 only)
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number of times a stripe requiring preread will be bypassed by
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a stripe that does not require preread. For fairness defaults
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to 1. Setting this to 0 disables bypass accounting and
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requires preread stripes to wait until all full-width stripe-
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writes are complete. Valid values are 0 to stripe_cache_size.
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