| ======= |
| 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>: |
| |
| ============= =============================================================== |
| raid0 RAID0 striping (no resilience) |
| raid1 RAID1 mirroring |
| raid4 RAID4 with dedicated last parity disk |
| raid5_n RAID5 with dedicated last parity disk supporting takeover |
| Same as raid4 |
| |
| - Transitory layout |
| 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 |
| raid6_n_6 RAID6 with dedicate parity disks |
| |
| - parity and Q-syndrome on the last 2 disks; |
| layout for takeover from/to raid4/raid5_n |
| raid6_la_6 Same as "raid_la" plus dedicated last Q-syndrome disk |
| |
| - layout for takeover from raid5_la from/to raid6 |
| raid6_ra_6 Same as "raid5_ra" dedicated last Q-syndrome disk |
| |
| - layout for takeover from raid5_ra from/to raid6 |
| raid6_ls_6 Same as "raid5_ls" dedicated last Q-syndrome disk |
| |
| - layout for takeover from raid5_ls from/to raid6 |
| raid6_rs_6 Same as "raid5_rs" dedicated last Q-syndrome disk |
| |
| - layout for takeover from raid5_rs from/to raid6 |
| raid10 Various RAID10 inspired algorithms chosen by additional params |
| (see raid10_format and raid10_copies below) |
| |
| - 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'. |
| |
| [delta_disks <N>] |
| The delta_disks option value (-251 < N < +251) triggers |
| device removal (negative value) or device addition (positive |
| value) to any reshape supporting raid levels 4/5/6 and 10. |
| RAID levels 4/5/6 allow for addition of devices (metadata |
| and data device tuple), raid10_near and raid10_offset only |
| allow for device addition. raid10_far does not support any |
| reshaping at all. |
| A minimum of devices have to be kept to enforce resilience, |
| which is 3 devices for raid4/5 and 4 devices for raid6. |
| |
| [data_offset <sectors>] |
| This option value defines the offset into each data device |
| where the data starts. This is used to provide out-of-place |
| reshaping space to avoid writing over data while |
| changing the layout of stripes, hence an interruption/crash |
| may happen at any time without the risk of losing data. |
| E.g. when adding devices to an existing raid set during |
| forward reshaping, the out-of-place space will be allocated |
| at the beginning of each raid device. The kernel raid4/5/6/10 |
| MD personalities supporting such device addition will read the data from |
| the existing first stripes (those with smaller number of stripes) |
| starting at data_offset to fill up a new stripe with the larger |
| number of stripes, calculate the redundancy blocks (CRC/Q-syndrome) |
| and write that new stripe to offset 0. Same will be applied to all |
| N-1 other new stripes. This out-of-place scheme is used to change |
| the RAID type (i.e. the allocation algorithm) as well, e.g. |
| changing from raid5_ls to raid5_n. |
| |
| [journal_dev <dev>] |
| This option adds a journal device to raid4/5/6 raid sets and |
| uses it to close the 'write hole' caused by the non-atomic updates |
| to the component devices which can cause data loss during recovery. |
| The journal device is used as writethrough thus causing writes to |
| be throttled versus non-journaled raid4/5/6 sets. |
| Takeover/reshape is not possible with a raid4/5/6 journal device; |
| it has to be deconfigured before requesting these. |
| |
| [journal_mode <mode>] |
| This option sets the caching mode on journaled raid4/5/6 raid sets |
| (see 'journal_dev <dev>' above) to 'writethrough' or 'writeback'. |
| If 'writeback' is selected the journal device has to be resilient |
| and must not suffer from the 'write hole' problem itself (e.g. use |
| raid1 or raid10) to avoid a single point of failure. |
| |
| <#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. A Maximum of 64 metadata/data device entries are supported |
| up to target version 1.8.0. |
| 1.9.0 supports up to 253 which is enforced by the used MD kernel runtime. |
| |
| 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. |
| <data_offset> The current data offset to the start of the user data on |
| each component device of a raid set (see the respective |
| raid parameter to support out-of-place reshaping). |
| <journal_char> - 'A' - active write-through journal device. |
| - 'a' - active write-back journal device. |
| - 'D' - dead journal device. |
| - '-' - no journal device. |
| =============== ========================================================= |
| |
| |
| 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. |
| ========= ================================================ |
| |
| |
| Discard Support |
| --------------- |
| The implementation of discard support among hardware vendors varies. |
| When a block is discarded, some storage devices will return zeroes when |
| the block is read. These devices set the 'discard_zeroes_data' |
| attribute. Other devices will return random data. Confusingly, some |
| devices that advertise 'discard_zeroes_data' will not reliably return |
| zeroes when discarded blocks are read! Since RAID 4/5/6 uses blocks |
| from a number of devices to calculate parity blocks and (for performance |
| reasons) relies on 'discard_zeroes_data' being reliable, it is important |
| that the devices be consistent. Blocks may be discarded in the middle |
| of a RAID 4/5/6 stripe and if subsequent read results are not |
| consistent, the parity blocks may be calculated differently at any time; |
| making the parity blocks useless for redundancy. It is important to |
| understand how your hardware behaves with discards if you are going to |
| enable discards with RAID 4/5/6. |
| |
| Since the behavior of storage devices is unreliable in this respect, |
| even when reporting 'discard_zeroes_data', by default RAID 4/5/6 |
| discard support is disabled -- this ensures data integrity at the |
| expense of losing some performance. |
| |
| Storage devices that properly support 'discard_zeroes_data' are |
| increasingly whitelisted in the kernel and can thus be trusted. |
| |
| For trusted devices, the following dm-raid module parameter can be set |
| to safely enable discard support for RAID 4/5/6: |
| |
| 'devices_handle_discards_safely' |
| |
| |
| 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". |
| 1.6.0 Add discard support (and devices_handle_discard_safely module param). |
| 1.7.0 Add support for MD RAID0 mappings. |
| 1.8.0 Explicitly check for compatible flags in the superblock metadata |
| and reject to start the raid set if any are set by a newer |
| target version, thus avoiding data corruption on a raid set |
| with a reshape in progress. |
| 1.9.0 Add support for RAID level takeover/reshape/region size |
| and set size reduction. |
| 1.9.1 Fix activation of existing RAID 4/10 mapped devices |
| 1.9.2 Don't emit '- -' on the status table line in case the constructor |
| fails reading a superblock. Correctly emit 'maj:min1 maj:min2' and |
| 'D' on the status line. If '- -' is passed into the constructor, emit |
| '- -' on the table line and '-' as the status line health character. |
| 1.10.0 Add support for raid4/5/6 journal device |
| 1.10.1 Fix data corruption on reshape request |
| 1.11.0 Fix table line argument order |
| (wrong raid10_copies/raid10_format sequence) |
| 1.11.1 Add raid4/5/6 journal write-back support via journal_mode option |
| 1.12.1 Fix for MD deadlock between mddev_suspend() and md_write_start() available |
| 1.13.0 Fix dev_health status at end of "recover" (was 'a', now 'A') |
| 1.13.1 Fix deadlock caused by early md_stop_writes(). Also fix size an |
| state races. |
| 1.13.2 Fix raid redundancy validation and avoid keeping raid set frozen |
| 1.14.0 Fix reshape race on small devices. Fix stripe adding reshape |
| deadlock/potential data corruption. Update superblock when |
| specific devices are requested via rebuild. Fix RAID leg |
| rebuild errors. |
| 1.15.0 Fix size extensions not being synchronized in case of new MD bitmap |
| pages allocated; also fix those not occuring after previous reductions |
