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Diffstat (limited to 'kernel/Documentation/device-mapper/dm-raid.txt')
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diff --git a/kernel/Documentation/device-mapper/dm-raid.txt b/kernel/Documentation/device-mapper/dm-raid.txt new file mode 100644 index 000000000..ef8ba9fa5 --- /dev/null +++ b/kernel/Documentation/device-mapper/dm-raid.txt @@ -0,0 +1,226 @@ +dm-raid +======= + +The device-mapper RAID (dm-raid) target provides a bridge from DM to MD. +It allows the MD RAID drivers to be accessed using a device-mapper +interface. + + +Mapping Table Interface +----------------------- +The target is named "raid" and it accepts the following parameters: + + <raid_type> <#raid_params> <raid_params> \ + <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>] + +<raid_type>: + raid1 RAID1 mirroring + raid4 RAID4 dedicated parity disk + raid5_la RAID5 left asymmetric + - rotating parity 0 with data continuation + raid5_ra RAID5 right asymmetric + - rotating parity N with data continuation + raid5_ls RAID5 left symmetric + - rotating parity 0 with data restart + raid5_rs RAID5 right symmetric + - rotating parity N with data restart + raid6_zr RAID6 zero restart + - rotating parity zero (left-to-right) with data restart + raid6_nr RAID6 N restart + - rotating parity N (right-to-left) with data restart + raid6_nc RAID6 N continue + - rotating parity N (right-to-left) with data continuation + raid10 Various RAID10 inspired algorithms chosen by additional params + - RAID10: Striped Mirrors (aka 'Striping on top of mirrors') + - RAID1E: Integrated Adjacent Stripe Mirroring + - RAID1E: Integrated Offset Stripe Mirroring + - and other similar RAID10 variants + + Reference: Chapter 4 of + http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf + +<#raid_params>: The number of parameters that follow. + +<raid_params> consists of + Mandatory parameters: + <chunk_size>: Chunk size in sectors. This parameter is often known as + "stripe size". It is the only mandatory parameter and + is placed first. + + followed by optional parameters (in any order): + [sync|nosync] Force or prevent RAID initialization. + + [rebuild <idx>] Rebuild drive number 'idx' (first drive is 0). + + [daemon_sleep <ms>] + Interval between runs of the bitmap daemon that + clear bits. A longer interval means less bitmap I/O but + resyncing after a failure is likely to take longer. + + [min_recovery_rate <kB/sec/disk>] Throttle RAID initialization + [max_recovery_rate <kB/sec/disk>] Throttle RAID initialization + [write_mostly <idx>] Mark drive index 'idx' write-mostly. + [max_write_behind <sectors>] See '--write-behind=' (man mdadm) + [stripe_cache <sectors>] Stripe cache size (RAID 4/5/6 only) + [region_size <sectors>] + The region_size multiplied by the number of regions is the + logical size of the array. The bitmap records the device + synchronisation state for each region. + + [raid10_copies <# copies>] + [raid10_format <near|far|offset>] + These two options are used to alter the default layout of + a RAID10 configuration. The number of copies is can be + specified, but the default is 2. There are also three + variations to how the copies are laid down - the default + is "near". Near copies are what most people think of with + respect to mirroring. If these options are left unspecified, + or 'raid10_copies 2' and/or 'raid10_format near' are given, + then the layouts for 2, 3 and 4 devices are: + 2 drives 3 drives 4 drives + -------- ---------- -------------- + A1 A1 A1 A1 A2 A1 A1 A2 A2 + A2 A2 A2 A3 A3 A3 A3 A4 A4 + A3 A3 A4 A4 A5 A5 A5 A6 A6 + A4 A4 A5 A6 A6 A7 A7 A8 A8 + .. .. .. .. .. .. .. .. .. + The 2-device layout is equivalent 2-way RAID1. The 4-device + layout is what a traditional RAID10 would look like. The + 3-device layout is what might be called a 'RAID1E - Integrated + Adjacent Stripe Mirroring'. + + If 'raid10_copies 2' and 'raid10_format far', then the layouts + for 2, 3 and 4 devices are: + 2 drives 3 drives 4 drives + -------- -------------- -------------------- + A1 A2 A1 A2 A3 A1 A2 A3 A4 + A3 A4 A4 A5 A6 A5 A6 A7 A8 + A5 A6 A7 A8 A9 A9 A10 A11 A12 + .. .. .. .. .. .. .. .. .. + A2 A1 A3 A1 A2 A2 A1 A4 A3 + A4 A3 A6 A4 A5 A6 A5 A8 A7 + A6 A5 A9 A7 A8 A10 A9 A12 A11 + .. .. .. .. .. .. .. .. .. + + If 'raid10_copies 2' and 'raid10_format offset', then the + layouts for 2, 3 and 4 devices are: + 2 drives 3 drives 4 drives + -------- ------------ ----------------- + A1 A2 A1 A2 A3 A1 A2 A3 A4 + A2 A1 A3 A1 A2 A2 A1 A4 A3 + A3 A4 A4 A5 A6 A5 A6 A7 A8 + A4 A3 A6 A4 A5 A6 A5 A8 A7 + A5 A6 A7 A8 A9 A9 A10 A11 A12 + A6 A5 A9 A7 A8 A10 A9 A12 A11 + .. .. .. .. .. .. .. .. .. + Here we see layouts closely akin to 'RAID1E - Integrated + Offset Stripe Mirroring'. + +<#raid_devs>: The number of devices composing the array. + Each device consists of two entries. The first is the device + containing the metadata (if any); the second is the one containing the + data. + + If a drive has failed or is missing at creation time, a '-' can be + given for both the metadata and data drives for a given position. + + +Example Tables +-------------- +# RAID4 - 4 data drives, 1 parity (no metadata devices) +# No metadata devices specified to hold superblock/bitmap info +# Chunk size of 1MiB +# (Lines separated for easy reading) + +0 1960893648 raid \ + raid4 1 2048 \ + 5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81 + +# RAID4 - 4 data drives, 1 parity (with metadata devices) +# Chunk size of 1MiB, force RAID initialization, +# min recovery rate at 20 kiB/sec/disk + +0 1960893648 raid \ + raid4 4 2048 sync min_recovery_rate 20 \ + 5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82 + + +Status Output +------------- +'dmsetup table' displays the table used to construct the mapping. +The optional parameters are always printed in the order listed +above with "sync" or "nosync" always output ahead of the other +arguments, regardless of the order used when originally loading the table. +Arguments that can be repeated are ordered by value. + + +'dmsetup status' yields information on the state and health of the array. +The output is as follows (normally a single line, but expanded here for +clarity): +1: <s> <l> raid \ +2: <raid_type> <#devices> <health_chars> \ +3: <sync_ratio> <sync_action> <mismatch_cnt> + +Line 1 is the standard output produced by device-mapper. +Line 2 & 3 are produced by the raid target and are best explained by example: + 0 1960893648 raid raid4 5 AAAAA 2/490221568 init 0 +Here we can see the RAID type is raid4, there are 5 devices - all of +which are 'A'live, and the array is 2/490221568 complete with its initial +recovery. Here is a fuller description of the individual fields: + <raid_type> Same as the <raid_type> used to create the array. + <health_chars> One char for each device, indicating: 'A' = alive and + in-sync, 'a' = alive but not in-sync, 'D' = dead/failed. + <sync_ratio> The ratio indicating how much of the array has undergone + the process described by 'sync_action'. If the + 'sync_action' is "check" or "repair", then the process + of "resync" or "recover" can be considered complete. + <sync_action> One of the following possible states: + idle - No synchronization action is being performed. + frozen - The current action has been halted. + resync - Array is undergoing its initial synchronization + or is resynchronizing after an unclean shutdown + (possibly aided by a bitmap). + recover - A device in the array is being rebuilt or + replaced. + check - A user-initiated full check of the array is + being performed. All blocks are read and + checked for consistency. The number of + discrepancies found are recorded in + <mismatch_cnt>. No changes are made to the + array by this action. + repair - The same as "check", but discrepancies are + corrected. + reshape - The array is undergoing a reshape. + <mismatch_cnt> The number of discrepancies found between mirror copies + in RAID1/10 or wrong parity values found in RAID4/5/6. + This value is valid only after a "check" of the array + is performed. A healthy array has a 'mismatch_cnt' of 0. + +Message Interface +----------------- +The dm-raid target will accept certain actions through the 'message' interface. +('man dmsetup' for more information on the message interface.) These actions +include: + "idle" - Halt the current sync action. + "frozen" - Freeze the current sync action. + "resync" - Initiate/continue a resync. + "recover"- Initiate/continue a recover process. + "check" - Initiate a check (i.e. a "scrub") of the array. + "repair" - Initiate a repair of the array. + "reshape"- Currently unsupported (-EINVAL). + +Version History +--------------- +1.0.0 Initial version. Support for RAID 4/5/6 +1.1.0 Added support for RAID 1 +1.2.0 Handle creation of arrays that contain failed devices. +1.3.0 Added support for RAID 10 +1.3.1 Allow device replacement/rebuild for RAID 10 +1.3.2 Fix/improve redundancy checking for RAID10 +1.4.0 Non-functional change. Removes arg from mapping function. +1.4.1 RAID10 fix redundancy validation checks (commit 55ebbb5). +1.4.2 Add RAID10 "far" and "offset" algorithm support. +1.5.0 Add message interface to allow manipulation of the sync_action. + New status (STATUSTYPE_INFO) fields: sync_action and mismatch_cnt. +1.5.1 Add ability to restore transiently failed devices on resume. +1.5.2 'mismatch_cnt' is zero unless [last_]sync_action is "check". |