| 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". |