766 lines
27 KiB
ReStructuredText
766 lines
27 KiB
ReStructuredText
RAID arrays
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===========
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Boot time assembly of RAID arrays
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---------------------------------
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Tools that manage md devices can be found at
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https://www.kernel.org/pub/linux/utils/raid/
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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.``
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+++++++++++++++++
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The number of the md device
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================= =========
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``md device no.`` device
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================= =========
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0 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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================= =========
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``raid level``
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++++++++++++++
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level of the RAID array
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=============== =============
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``raid level`` level
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=============== =============
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-1 linear mode
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0 striped mode
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=============== =============
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other modes are only supported with persistent super blocks
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``chunk size factor``
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+++++++++++++++++++++
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(raid-0 and raid-1 only)
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Set the chunk size as 4k << n.
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``fault level``
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+++++++++++++++
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Totally ignored
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``dev0`` to ``devn``
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++++++++++++++++++++
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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 5 or 6, 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 raid 1, parity
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calculation in raid 4/5).
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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 be passed 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 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 have ``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 its 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 this will contain
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the new number of devices.
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Some raid levels allow this value to be set while the array is
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active. This will reconfigure the array. Otherwise it can only
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be set while assembling an array.
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A change to this attribute will not be permitted if it would
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reduce the size of the array. To reduce the number of drives
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in an e.g. raid5, the array size must first be reduced by
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setting the ``array_size`` attribute.
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chunk_size
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This is the size in bytes for ``chunks`` and is only relevant to
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raid levels that involve striping (0,4,5,6,10). 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 interpreted differently by different
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levels. It can be written while assembling an array.
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array_size
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This can be used to artificially constrain the available space in
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the array to be less than is actually available on the combined
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devices. Writing a number (in Kilobytes) which is less than
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the available size will set the size. Any reconfiguration of the
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array (e.g. adding devices) will not cause the size to change.
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Writing the word ``default`` will cause the effective size of the
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array to be whatever size is actually available based on
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``level``, ``chunk_size`` and ``component_size``.
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This can be used to reduce the size of the array before reducing
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the number of devices in a raid4/5/6, or to support external
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metadata formats which mandate such clipping.
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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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Alternately it can be ``external:`` followed by a string which
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is set by user-space. This indicates that metadata is managed
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by a user-space program. Any device failure or other event that
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requires a metadata update will cause array activity to be
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suspended until the event is acknowledged.
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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 (or ``none`` since 2.6.30-rc1). At
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array creation it will default to 0, though starting the array as
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``clean`` will 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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Select/poll works on this file. All changes except between
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Active_idle and active (which can be frequent and are not
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very interesting) are notified. active->active_idle is
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reported if the metadata is externally managed.
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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 quiescent
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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
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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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bitmap/location
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This indicates where the write-intent bitmap for the array is
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stored.
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It can be one of ``none``, ``file`` or ``[+-]N``.
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``file`` may later be extended to ``file:/file/name``
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``[+-]N`` means that many sectors from the start of the metadata.
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This is replicated on all devices. For arrays with externally
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managed metadata, the offset is from the beginning of the
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device.
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bitmap/chunksize
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The size, in bytes, of the chunk which will be represented by a
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single bit. For RAID456, it is a portion of an individual
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device. For RAID10, it is a portion of the array. For RAID1, it
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is both (they come to the same thing).
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bitmap/time_base
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The time, in seconds, between looking for bits in the bitmap to
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be cleared. In the current implementation, a bit will be cleared
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between 2 and 3 times ``time_base`` after all the covered blocks
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are known to be in-sync.
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bitmap/backlog
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When write-mostly devices are active in a RAID1, write requests
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to those devices proceed in the background - the filesystem (or
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other user of the device) does not have to wait for them.
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``backlog`` sets a limit on the number of concurrent background
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writes. If there are more than this, new writes will by
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synchronous.
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bitmap/metadata
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This can be either ``internal`` or ``external``.
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``internal``
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is the default and means the metadata for the bitmap
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is stored in the first 256 bytes of the allocated space and is
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managed by the md module.
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``external``
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means that bitmap metadata is managed externally to
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the kernel (i.e. by some userspace program)
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bitmap/can_clear
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This is either ``true`` or ``false``. If ``true``, then bits in the
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bitmap will be cleared when the corresponding blocks are thought
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to be in-sync. If ``false``, bits will never be cleared.
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This is automatically set to ``false`` if a write happens on a
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degraded array, or if the array becomes degraded during a write.
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When metadata is managed externally, it should be set to true
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once the array becomes non-degraded, and this fact has been
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recorded in the metadata.
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consistency_policy
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This indicates how the array maintains consistency in case of unexpected
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shutdown. It can be:
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none
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Array has no redundancy information, e.g. raid0, linear.
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resync
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Full resync is performed and all redundancy is regenerated when the
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array is started after unclean shutdown.
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bitmap
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Resync assisted by a write-intent bitmap.
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journal
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For raid4/5/6, journal device is used to log transactions and replay
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after unclean shutdown.
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ppl
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For raid5 only, Partial Parity Log is used to close the write hole and
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eliminate resync.
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The accepted values when writing to this file are ``ppl`` and ``resync``,
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used to enable and disable PPL.
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uuid
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This indicates the UUID of the array in the following format:
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xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
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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
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device has been kicked from active use due to
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a detected fault, or it has unacknowledged bad
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blocks
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in_sync
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device is a fully in-sync member of the array
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writemostly
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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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blocked
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device has failed, and the failure hasn't been
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acknowledged yet by the metadata handler.
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Writes that would write to this device if
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it were not faulty are blocked.
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spare
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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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write_error
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device has ever seen a write error.
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want_replacement
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device is (mostly) working but probably
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should be replaced, either due to errors or
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due to user request.
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replacement
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device is a replacement for another active
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device with same raid_disk.
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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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Writing ``blocked`` sets the ``blocked`` flag.
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Writing ``-blocked`` clears the ``blocked`` flags and allows writes
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to complete and possibly simulates an error.
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Writing ``in_sync`` sets the in_sync flag.
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Writing ``write_error`` sets writeerrorseen flag.
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Writing ``-write_error`` clears writeerrorseen flag.
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Writing ``want_replacement`` is allowed at any time except to a
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replacement device or a spare. It sets the flag.
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Writing ``-want_replacement`` is allowed at any time. It clears
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the flag.
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Writing ``replacement`` or ``-replacement`` is only allowed before
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starting the array. It sets or clears the flag.
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This file responds to select/poll. Any change to ``faulty``
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or ``blocked`` causes an event.
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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
|
|
array. A device for which this is set is assumed to be working.
|
|
|
|
offset
|
|
This gives the location in the device (in sectors from the
|
|
start) where data from the array will be stored. Any part of
|
|
the device before this offset is not touched, unless it is
|
|
used for storing metadata (Formats 1.1 and 1.2).
|
|
|
|
size
|
|
The amount of the device, after the offset, that can be used
|
|
for storage of data. This will normally be the same as the
|
|
component_size. This can be written while assembling an
|
|
array. If a value less than the current component_size is
|
|
written, it will be rejected.
|
|
|
|
recovery_start
|
|
When the device is not ``in_sync``, this records the number of
|
|
sectors from the start of the device which are known to be
|
|
correct. This is normally zero, but during a recovery
|
|
operation it will steadily increase, and if the recovery is
|
|
interrupted, restoring this value can cause recovery to
|
|
avoid repeating the earlier blocks. With v1.x metadata, this
|
|
value is saved and restored automatically.
|
|
|
|
This can be set whenever the device is not an active member of
|
|
the array, either before the array is activated, or before
|
|
the ``slot`` is set.
|
|
|
|
Setting this to ``none`` is equivalent to setting ``in_sync``.
|
|
Setting to any other value also clears the ``in_sync`` flag.
|
|
|
|
bad_blocks
|
|
This gives the list of all known bad blocks in the form of
|
|
start address and length (in sectors respectively). If output
|
|
is too big to fit in a page, it will be truncated. Writing
|
|
``sector length`` to this file adds new acknowledged (i.e.
|
|
recorded to disk safely) bad blocks.
|
|
|
|
unacknowledged_bad_blocks
|
|
This gives the list of known-but-not-yet-saved-to-disk bad
|
|
blocks in the same form of ``bad_blocks``. If output is too big
|
|
to fit in a page, it will be truncated. Writing to this file
|
|
adds bad blocks without acknowledging them. This is largely
|
|
for testing.
|
|
|
|
ppl_sector, ppl_size
|
|
Location and size (in sectors) of the space used for Partial Parity Log
|
|
on this device.
|
|
|
|
|
|
An active md device will also contain an entry for each active device
|
|
in the array. These are named::
|
|
|
|
rdNN
|
|
|
|
where ``NN`` is the position 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 (1,4,5,6,10)
|
|
also have
|
|
|
|
sync_action
|
|
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 blocks and checks
|
|
them. A repair may also happen for some raid
|
|
levels.
|
|
|
|
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
|
|
this.
|
|
|
|
``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``.
|
|
|
|
This file responds to select/poll. Any important change in the value
|
|
triggers a poll event. Sometimes the value will briefly be
|
|
``recover`` if a recovery seems to be needed, but cannot be
|
|
achieved. In that case, the transition to ``recover`` isn't
|
|
notified, but the transition away is.
|
|
|
|
degraded
|
|
This contains a count of the number of devices by which the
|
|
arrays is degraded. So an optimal array will show ``0``. A
|
|
single failed/missing drive will show ``1``, etc.
|
|
|
|
This file responds to select/poll, any increase or decrease
|
|
in the count of missing devices will trigger an event.
|
|
|
|
mismatch_count
|
|
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 may be larger than the number of actual errors
|
|
by a factor of the number of sectors in a page.
|
|
|
|
bitmap_set_bits
|
|
If the array has a write-intent bitmap, then writing to this
|
|
attribute can set bits in the bitmap, indicating that a resync
|
|
would need to check the corresponding blocks. Either individual
|
|
numbers or start-end pairs can be written. Multiple numbers
|
|
can be separated by a space.
|
|
|
|
Note that the numbers are ``bit`` numbers, not ``block`` numbers.
|
|
They should be scaled by the bitmap_chunksize.
|
|
|
|
sync_speed_min, sync_speed_max
|
|
This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}``
|
|
however they only apply to the particular array.
|
|
|
|
If no value has been written to these, or if the word ``system``
|
|
is written, then the system-wide value is used. If a value,
|
|
in kibibytes-per-second is written, then it is used.
|
|
|
|
When the files are read, they show the currently active value
|
|
followed by ``(local)`` or ``(system)`` depending on whether it is
|
|
a locally set or system-wide value.
|
|
|
|
sync_completed
|
|
This shows the number of sectors that have been completed of
|
|
whatever the current sync_action is, followed by the number of
|
|
sectors in total that could need to be processed. The two
|
|
numbers are separated by a ``/`` thus effectively showing one
|
|
value, a fraction of the process that is complete.
|
|
|
|
A ``select`` on this attribute will return when resync completes,
|
|
when it reaches the current sync_max (below) and possibly at
|
|
other times.
|
|
|
|
sync_speed
|
|
This shows the current actual speed, in K/sec, of the current
|
|
sync_action. It is averaged over the last 30 seconds.
|
|
|
|
suspend_lo, suspend_hi
|
|
The two values, given as numbers of sectors, indicate a range
|
|
within the array where IO will be blocked. This is currently
|
|
only supported for raid4/5/6.
|
|
|
|
sync_min, sync_max
|
|
The two values, given as numbers of sectors, indicate a range
|
|
within the array where ``check``/``repair`` will operate. Must be
|
|
a multiple of chunk_size. When it reaches ``sync_max`` it will
|
|
pause, rather than complete.
|
|
You can use ``select`` or ``poll`` on ``sync_completed`` to wait for
|
|
that number to reach sync_max. Then you can either increase
|
|
``sync_max``, or can write ``idle`` to ``sync_action``.
|
|
|
|
The value of ``max`` for ``sync_max`` effectively disables the limit.
|
|
When a resync is active, the value can only ever be increased,
|
|
never decreased.
|
|
The value of ``0`` is the minimum for ``sync_min``.
|
|
|
|
|
|
|
|
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, 17). Default is 256.
|
|
|
|
strip_cache_active (currently raid5 only)
|
|
number of active entries in the stripe cache
|
|
|
|
preread_bypass_threshold (currently raid5 only)
|
|
number of times a stripe requiring preread will be bypassed by
|
|
a stripe that does not require preread. For fairness defaults
|
|
to 1. Setting this to 0 disables bypass accounting and
|
|
requires preread stripes to wait until all full-width stripe-
|
|
writes are complete. Valid values are 0 to stripe_cache_size.
|
|
|
|
journal_mode (currently raid5 only)
|
|
The cache mode for raid5. raid5 could include an extra disk for
|
|
caching. The mode can be "write-throuth" and "write-back". The
|
|
default is "write-through".
|
|
|
|
ppl_write_hint
|
|
NVMe stream ID to be set for each PPL write request.
|