346 lines
16 KiB
Plaintext
346 lines
16 KiB
Plaintext
dm-raid
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=======
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The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.
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It allows the MD RAID drivers to be accessed using a device-mapper
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interface.
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Mapping Table Interface
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-----------------------
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The target is named "raid" and it accepts the following parameters:
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<raid_type> <#raid_params> <raid_params> \
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<#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]
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<raid_type>:
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raid0 RAID0 striping (no resilience)
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raid1 RAID1 mirroring
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raid4 RAID4 with dedicated last parity disk
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raid5_n RAID5 with dedicated last parity disk supporting takeover
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Same as raid4
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-Transitory layout
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raid5_la RAID5 left asymmetric
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- rotating parity 0 with data continuation
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raid5_ra RAID5 right asymmetric
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- rotating parity N with data continuation
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raid5_ls RAID5 left symmetric
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- rotating parity 0 with data restart
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raid5_rs RAID5 right symmetric
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- rotating parity N with data restart
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raid6_zr RAID6 zero restart
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- rotating parity zero (left-to-right) with data restart
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raid6_nr RAID6 N restart
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- rotating parity N (right-to-left) with data restart
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raid6_nc RAID6 N continue
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- rotating parity N (right-to-left) with data continuation
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raid6_n_6 RAID6 with dedicate parity disks
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- parity and Q-syndrome on the last 2 disks;
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layout for takeover from/to raid4/raid5_n
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raid6_la_6 Same as "raid_la" plus dedicated last Q-syndrome disk
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- layout for takeover from raid5_la from/to raid6
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raid6_ra_6 Same as "raid5_ra" dedicated last Q-syndrome disk
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- layout for takeover from raid5_ra from/to raid6
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raid6_ls_6 Same as "raid5_ls" dedicated last Q-syndrome disk
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- layout for takeover from raid5_ls from/to raid6
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raid6_rs_6 Same as "raid5_rs" dedicated last Q-syndrome disk
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- layout for takeover from raid5_rs from/to raid6
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raid10 Various RAID10 inspired algorithms chosen by additional params
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(see raid10_format and raid10_copies below)
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- RAID10: Striped Mirrors (aka 'Striping on top of mirrors')
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- RAID1E: Integrated Adjacent Stripe Mirroring
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- RAID1E: Integrated Offset Stripe Mirroring
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- and other similar RAID10 variants
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Reference: Chapter 4 of
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http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
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<#raid_params>: The number of parameters that follow.
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<raid_params> consists of
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Mandatory parameters:
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<chunk_size>: Chunk size in sectors. This parameter is often known as
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"stripe size". It is the only mandatory parameter and
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is placed first.
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followed by optional parameters (in any order):
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[sync|nosync] Force or prevent RAID initialization.
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[rebuild <idx>] Rebuild drive number 'idx' (first drive is 0).
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[daemon_sleep <ms>]
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Interval between runs of the bitmap daemon that
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clear bits. A longer interval means less bitmap I/O but
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resyncing after a failure is likely to take longer.
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[min_recovery_rate <kB/sec/disk>] Throttle RAID initialization
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[max_recovery_rate <kB/sec/disk>] Throttle RAID initialization
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[write_mostly <idx>] Mark drive index 'idx' write-mostly.
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[max_write_behind <sectors>] See '--write-behind=' (man mdadm)
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[stripe_cache <sectors>] Stripe cache size (RAID 4/5/6 only)
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[region_size <sectors>]
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The region_size multiplied by the number of regions is the
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logical size of the array. The bitmap records the device
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synchronisation state for each region.
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[raid10_copies <# copies>]
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[raid10_format <near|far|offset>]
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These two options are used to alter the default layout of
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a RAID10 configuration. The number of copies is can be
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specified, but the default is 2. There are also three
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variations to how the copies are laid down - the default
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is "near". Near copies are what most people think of with
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respect to mirroring. If these options are left unspecified,
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or 'raid10_copies 2' and/or 'raid10_format near' are given,
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then the layouts for 2, 3 and 4 devices are:
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2 drives 3 drives 4 drives
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-------- ---------- --------------
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A1 A1 A1 A1 A2 A1 A1 A2 A2
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A2 A2 A2 A3 A3 A3 A3 A4 A4
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A3 A3 A4 A4 A5 A5 A5 A6 A6
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A4 A4 A5 A6 A6 A7 A7 A8 A8
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.. .. .. .. .. .. .. .. ..
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The 2-device layout is equivalent 2-way RAID1. The 4-device
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layout is what a traditional RAID10 would look like. The
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3-device layout is what might be called a 'RAID1E - Integrated
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Adjacent Stripe Mirroring'.
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If 'raid10_copies 2' and 'raid10_format far', then the layouts
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for 2, 3 and 4 devices are:
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2 drives 3 drives 4 drives
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-------- -------------- --------------------
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A1 A2 A1 A2 A3 A1 A2 A3 A4
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A3 A4 A4 A5 A6 A5 A6 A7 A8
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A5 A6 A7 A8 A9 A9 A10 A11 A12
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.. .. .. .. .. .. .. .. ..
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A2 A1 A3 A1 A2 A2 A1 A4 A3
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A4 A3 A6 A4 A5 A6 A5 A8 A7
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A6 A5 A9 A7 A8 A10 A9 A12 A11
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.. .. .. .. .. .. .. .. ..
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If 'raid10_copies 2' and 'raid10_format offset', then the
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layouts for 2, 3 and 4 devices are:
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2 drives 3 drives 4 drives
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-------- ------------ -----------------
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A1 A2 A1 A2 A3 A1 A2 A3 A4
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A2 A1 A3 A1 A2 A2 A1 A4 A3
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A3 A4 A4 A5 A6 A5 A6 A7 A8
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A4 A3 A6 A4 A5 A6 A5 A8 A7
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A5 A6 A7 A8 A9 A9 A10 A11 A12
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A6 A5 A9 A7 A8 A10 A9 A12 A11
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.. .. .. .. .. .. .. .. ..
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Here we see layouts closely akin to 'RAID1E - Integrated
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Offset Stripe Mirroring'.
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[delta_disks <N>]
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The delta_disks option value (-251 < N < +251) triggers
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device removal (negative value) or device addition (positive
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value) to any reshape supporting raid levels 4/5/6 and 10.
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RAID levels 4/5/6 allow for addition of devices (metadata
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and data device tuple), raid10_near and raid10_offset only
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allow for device addition. raid10_far does not support any
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reshaping at all.
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A minimum of devices have to be kept to enforce resilience,
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which is 3 devices for raid4/5 and 4 devices for raid6.
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[data_offset <sectors>]
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This option value defines the offset into each data device
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where the data starts. This is used to provide out-of-place
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reshaping space to avoid writing over data whilst
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changing the layout of stripes, hence an interruption/crash
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may happen at any time without the risk of losing data.
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E.g. when adding devices to an existing raid set during
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forward reshaping, the out-of-place space will be allocated
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at the beginning of each raid device. The kernel raid4/5/6/10
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MD personalities supporting such device addition will read the data from
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the existing first stripes (those with smaller number of stripes)
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starting at data_offset to fill up a new stripe with the larger
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number of stripes, calculate the redundancy blocks (CRC/Q-syndrome)
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and write that new stripe to offset 0. Same will be applied to all
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N-1 other new stripes. This out-of-place scheme is used to change
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the RAID type (i.e. the allocation algorithm) as well, e.g.
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changing from raid5_ls to raid5_n.
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[journal_dev <dev>]
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This option adds a journal device to raid4/5/6 raid sets and
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uses it to close the 'write hole' caused by the non-atomic updates
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to the component devices which can cause data loss during recovery.
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The journal device is used as writethrough thus causing writes to
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be throttled versus non-journaled raid4/5/6 sets.
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Takeover/reshape is not possible with a raid4/5/6 journal device;
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it has to be deconfigured before requesting these.
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[journal_mode <mode>]
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This option sets the caching mode on journaled raid4/5/6 raid sets
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(see 'journal_dev <dev>' above) to 'writethrough' or 'writeback'.
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If 'writeback' is selected the journal device has to be resilient
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and must not suffer from the 'write hole' problem itself (e.g. use
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raid1 or raid10) to avoid a single point of failure.
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<#raid_devs>: The number of devices composing the array.
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Each device consists of two entries. The first is the device
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containing the metadata (if any); the second is the one containing the
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data. A Maximum of 64 metadata/data device entries are supported
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up to target version 1.8.0.
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1.9.0 supports up to 253 which is enforced by the used MD kernel runtime.
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If a drive has failed or is missing at creation time, a '-' can be
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given for both the metadata and data drives for a given position.
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Example Tables
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--------------
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# RAID4 - 4 data drives, 1 parity (no metadata devices)
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# No metadata devices specified to hold superblock/bitmap info
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# Chunk size of 1MiB
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# (Lines separated for easy reading)
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0 1960893648 raid \
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raid4 1 2048 \
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5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81
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# RAID4 - 4 data drives, 1 parity (with metadata devices)
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# Chunk size of 1MiB, force RAID initialization,
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# min recovery rate at 20 kiB/sec/disk
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0 1960893648 raid \
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raid4 4 2048 sync min_recovery_rate 20 \
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5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82
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Status Output
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-------------
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'dmsetup table' displays the table used to construct the mapping.
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The optional parameters are always printed in the order listed
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above with "sync" or "nosync" always output ahead of the other
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arguments, regardless of the order used when originally loading the table.
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Arguments that can be repeated are ordered by value.
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'dmsetup status' yields information on the state and health of the array.
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The output is as follows (normally a single line, but expanded here for
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clarity):
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1: <s> <l> raid \
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2: <raid_type> <#devices> <health_chars> \
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3: <sync_ratio> <sync_action> <mismatch_cnt>
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Line 1 is the standard output produced by device-mapper.
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Line 2 & 3 are produced by the raid target and are best explained by example:
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0 1960893648 raid raid4 5 AAAAA 2/490221568 init 0
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Here we can see the RAID type is raid4, there are 5 devices - all of
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which are 'A'live, and the array is 2/490221568 complete with its initial
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recovery. Here is a fuller description of the individual fields:
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<raid_type> Same as the <raid_type> used to create the array.
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<health_chars> One char for each device, indicating: 'A' = alive and
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in-sync, 'a' = alive but not in-sync, 'D' = dead/failed.
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<sync_ratio> The ratio indicating how much of the array has undergone
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the process described by 'sync_action'. If the
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'sync_action' is "check" or "repair", then the process
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of "resync" or "recover" can be considered complete.
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<sync_action> One of the following possible states:
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idle - No synchronization action is being performed.
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frozen - The current action has been halted.
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resync - Array is undergoing its initial synchronization
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or is resynchronizing after an unclean shutdown
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(possibly aided by a bitmap).
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recover - A device in the array is being rebuilt or
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replaced.
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check - A user-initiated full check of the array is
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being performed. All blocks are read and
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checked for consistency. The number of
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discrepancies found are recorded in
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<mismatch_cnt>. No changes are made to the
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array by this action.
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repair - The same as "check", but discrepancies are
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corrected.
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reshape - The array is undergoing a reshape.
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<mismatch_cnt> The number of discrepancies found between mirror copies
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in RAID1/10 or wrong parity values found in RAID4/5/6.
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This value is valid only after a "check" of the array
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is performed. A healthy array has a 'mismatch_cnt' of 0.
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<data_offset> The current data offset to the start of the user data on
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each component device of a raid set (see the respective
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raid parameter to support out-of-place reshaping).
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<journal_char> 'A' - active write-through journal device.
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'a' - active write-back journal device.
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'D' - dead journal device.
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'-' - no journal device.
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Message Interface
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-----------------
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The dm-raid target will accept certain actions through the 'message' interface.
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('man dmsetup' for more information on the message interface.) These actions
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include:
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"idle" - Halt the current sync action.
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"frozen" - Freeze the current sync action.
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"resync" - Initiate/continue a resync.
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"recover"- Initiate/continue a recover process.
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"check" - Initiate a check (i.e. a "scrub") of the array.
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"repair" - Initiate a repair of the array.
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Discard Support
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---------------
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The implementation of discard support among hardware vendors varies.
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When a block is discarded, some storage devices will return zeroes when
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the block is read. These devices set the 'discard_zeroes_data'
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attribute. Other devices will return random data. Confusingly, some
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devices that advertise 'discard_zeroes_data' will not reliably return
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zeroes when discarded blocks are read! Since RAID 4/5/6 uses blocks
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from a number of devices to calculate parity blocks and (for performance
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reasons) relies on 'discard_zeroes_data' being reliable, it is important
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that the devices be consistent. Blocks may be discarded in the middle
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of a RAID 4/5/6 stripe and if subsequent read results are not
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consistent, the parity blocks may be calculated differently at any time;
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making the parity blocks useless for redundancy. It is important to
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understand how your hardware behaves with discards if you are going to
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enable discards with RAID 4/5/6.
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Since the behavior of storage devices is unreliable in this respect,
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even when reporting 'discard_zeroes_data', by default RAID 4/5/6
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discard support is disabled -- this ensures data integrity at the
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expense of losing some performance.
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Storage devices that properly support 'discard_zeroes_data' are
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increasingly whitelisted in the kernel and can thus be trusted.
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For trusted devices, the following dm-raid module parameter can be set
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to safely enable discard support for RAID 4/5/6:
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'devices_handle_discards_safely'
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Version History
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---------------
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1.0.0 Initial version. Support for RAID 4/5/6
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1.1.0 Added support for RAID 1
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1.2.0 Handle creation of arrays that contain failed devices.
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1.3.0 Added support for RAID 10
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1.3.1 Allow device replacement/rebuild for RAID 10
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1.3.2 Fix/improve redundancy checking for RAID10
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1.4.0 Non-functional change. Removes arg from mapping function.
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1.4.1 RAID10 fix redundancy validation checks (commit 55ebbb5).
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1.4.2 Add RAID10 "far" and "offset" algorithm support.
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1.5.0 Add message interface to allow manipulation of the sync_action.
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New status (STATUSTYPE_INFO) fields: sync_action and mismatch_cnt.
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1.5.1 Add ability to restore transiently failed devices on resume.
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1.5.2 'mismatch_cnt' is zero unless [last_]sync_action is "check".
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1.6.0 Add discard support (and devices_handle_discard_safely module param).
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1.7.0 Add support for MD RAID0 mappings.
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1.8.0 Explicitly check for compatible flags in the superblock metadata
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and reject to start the raid set if any are set by a newer
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target version, thus avoiding data corruption on a raid set
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with a reshape in progress.
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1.9.0 Add support for RAID level takeover/reshape/region size
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and set size reduction.
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1.9.1 Fix activation of existing RAID 4/10 mapped devices
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1.9.2 Don't emit '- -' on the status table line in case the constructor
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fails reading a superblock. Correctly emit 'maj:min1 maj:min2' and
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'D' on the status line. If '- -' is passed into the constructor, emit
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'- -' on the table line and '-' as the status line health character.
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1.10.0 Add support for raid4/5/6 journal device
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1.10.1 Fix data corruption on reshape request
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1.11.0 Fix table line argument order
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(wrong raid10_copies/raid10_format sequence)
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1.11.1 Add raid4/5/6 journal write-back support via journal_mode option
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