424 lines · plain
1=======2dm-raid3=======4 5The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.6It allows the MD RAID drivers to be accessed using a device-mapper7interface.8 9 10Mapping Table Interface11-----------------------12The target is named "raid" and it accepts the following parameters::13 14 <raid_type> <#raid_params> <raid_params> \15 <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]16 17<raid_type>:18 19 ============= ===============================================================20 raid0 RAID0 striping (no resilience)21 raid1 RAID1 mirroring22 raid4 RAID4 with dedicated last parity disk23 raid5_n RAID5 with dedicated last parity disk supporting takeover24 Same as raid425 26 - Transitory layout27 raid5_la RAID5 left asymmetric28 29 - rotating parity 0 with data continuation30 raid5_ra RAID5 right asymmetric31 32 - rotating parity N with data continuation33 raid5_ls RAID5 left symmetric34 35 - rotating parity 0 with data restart36 raid5_rs RAID5 right symmetric37 38 - rotating parity N with data restart39 raid6_zr RAID6 zero restart40 41 - rotating parity zero (left-to-right) with data restart42 raid6_nr RAID6 N restart43 44 - rotating parity N (right-to-left) with data restart45 raid6_nc RAID6 N continue46 47 - rotating parity N (right-to-left) with data continuation48 raid6_n_6 RAID6 with dedicate parity disks49 50 - parity and Q-syndrome on the last 2 disks;51 layout for takeover from/to raid4/raid5_n52 raid6_la_6 Same as "raid_la" plus dedicated last Q-syndrome disk53 54 - layout for takeover from raid5_la from/to raid655 raid6_ra_6 Same as "raid5_ra" dedicated last Q-syndrome disk56 57 - layout for takeover from raid5_ra from/to raid658 raid6_ls_6 Same as "raid5_ls" dedicated last Q-syndrome disk59 60 - layout for takeover from raid5_ls from/to raid661 raid6_rs_6 Same as "raid5_rs" dedicated last Q-syndrome disk62 63 - layout for takeover from raid5_rs from/to raid664 raid10 Various RAID10 inspired algorithms chosen by additional params65 (see raid10_format and raid10_copies below)66 67 - RAID10: Striped Mirrors (aka 'Striping on top of mirrors')68 - RAID1E: Integrated Adjacent Stripe Mirroring69 - RAID1E: Integrated Offset Stripe Mirroring70 - and other similar RAID10 variants71 ============= ===============================================================72 73 Reference: Chapter 4 of74 https://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf75 76<#raid_params>: The number of parameters that follow.77 78<raid_params> consists of79 80 Mandatory parameters:81 <chunk_size>:82 Chunk size in sectors. This parameter is often known as83 "stripe size". It is the only mandatory parameter and84 is placed first.85 86 followed by optional parameters (in any order):87 [sync|nosync]88 Force or prevent RAID initialization.89 90 [rebuild <idx>]91 Rebuild drive number 'idx' (first drive is 0).92 93 [daemon_sleep <ms>]94 Interval between runs of the bitmap daemon that95 clear bits. A longer interval means less bitmap I/O but96 resyncing after a failure is likely to take longer.97 98 [min_recovery_rate <kB/sec/disk>]99 Throttle RAID initialization100 [max_recovery_rate <kB/sec/disk>]101 Throttle RAID initialization102 [write_mostly <idx>]103 Mark drive index 'idx' write-mostly.104 [max_write_behind <sectors>]105 See '--write-behind=' (man mdadm)106 [stripe_cache <sectors>]107 Stripe cache size (RAID 4/5/6 only)108 [region_size <sectors>]109 The region_size multiplied by the number of regions is the110 logical size of the array. The bitmap records the device111 synchronisation state for each region.112 113 [raid10_copies <# copies>], [raid10_format <near|far|offset>]114 These two options are used to alter the default layout of115 a RAID10 configuration. The number of copies is can be116 specified, but the default is 2. There are also three117 variations to how the copies are laid down - the default118 is "near". Near copies are what most people think of with119 respect to mirroring. If these options are left unspecified,120 or 'raid10_copies 2' and/or 'raid10_format near' are given,121 then the layouts for 2, 3 and 4 devices are:122 123 ======== ========== ==============124 2 drives 3 drives 4 drives125 ======== ========== ==============126 A1 A1 A1 A1 A2 A1 A1 A2 A2127 A2 A2 A2 A3 A3 A3 A3 A4 A4128 A3 A3 A4 A4 A5 A5 A5 A6 A6129 A4 A4 A5 A6 A6 A7 A7 A8 A8130 .. .. .. .. .. .. .. .. ..131 ======== ========== ==============132 133 The 2-device layout is equivalent 2-way RAID1. The 4-device134 layout is what a traditional RAID10 would look like. The135 3-device layout is what might be called a 'RAID1E - Integrated136 Adjacent Stripe Mirroring'.137 138 If 'raid10_copies 2' and 'raid10_format far', then the layouts139 for 2, 3 and 4 devices are:140 141 ======== ============ ===================142 2 drives 3 drives 4 drives143 ======== ============ ===================144 A1 A2 A1 A2 A3 A1 A2 A3 A4145 A3 A4 A4 A5 A6 A5 A6 A7 A8146 A5 A6 A7 A8 A9 A9 A10 A11 A12147 .. .. .. .. .. .. .. .. ..148 A2 A1 A3 A1 A2 A2 A1 A4 A3149 A4 A3 A6 A4 A5 A6 A5 A8 A7150 A6 A5 A9 A7 A8 A10 A9 A12 A11151 .. .. .. .. .. .. .. .. ..152 ======== ============ ===================153 154 If 'raid10_copies 2' and 'raid10_format offset', then the155 layouts for 2, 3 and 4 devices are:156 157 ======== ========== ================158 2 drives 3 drives 4 drives159 ======== ========== ================160 A1 A2 A1 A2 A3 A1 A2 A3 A4161 A2 A1 A3 A1 A2 A2 A1 A4 A3162 A3 A4 A4 A5 A6 A5 A6 A7 A8163 A4 A3 A6 A4 A5 A6 A5 A8 A7164 A5 A6 A7 A8 A9 A9 A10 A11 A12165 A6 A5 A9 A7 A8 A10 A9 A12 A11166 .. .. .. .. .. .. .. .. ..167 ======== ========== ================168 169 Here we see layouts closely akin to 'RAID1E - Integrated170 Offset Stripe Mirroring'.171 172 [delta_disks <N>]173 The delta_disks option value (-251 < N < +251) triggers174 device removal (negative value) or device addition (positive175 value) to any reshape supporting raid levels 4/5/6 and 10.176 RAID levels 4/5/6 allow for addition of devices (metadata177 and data device tuple), raid10_near and raid10_offset only178 allow for device addition. raid10_far does not support any179 reshaping at all.180 A minimum of devices have to be kept to enforce resilience,181 which is 3 devices for raid4/5 and 4 devices for raid6.182 183 [data_offset <sectors>]184 This option value defines the offset into each data device185 where the data starts. This is used to provide out-of-place186 reshaping space to avoid writing over data while187 changing the layout of stripes, hence an interruption/crash188 may happen at any time without the risk of losing data.189 E.g. when adding devices to an existing raid set during190 forward reshaping, the out-of-place space will be allocated191 at the beginning of each raid device. The kernel raid4/5/6/10192 MD personalities supporting such device addition will read the data from193 the existing first stripes (those with smaller number of stripes)194 starting at data_offset to fill up a new stripe with the larger195 number of stripes, calculate the redundancy blocks (CRC/Q-syndrome)196 and write that new stripe to offset 0. Same will be applied to all197 N-1 other new stripes. This out-of-place scheme is used to change198 the RAID type (i.e. the allocation algorithm) as well, e.g.199 changing from raid5_ls to raid5_n.200 201 [journal_dev <dev>]202 This option adds a journal device to raid4/5/6 raid sets and203 uses it to close the 'write hole' caused by the non-atomic updates204 to the component devices which can cause data loss during recovery.205 The journal device is used as writethrough thus causing writes to206 be throttled versus non-journaled raid4/5/6 sets.207 Takeover/reshape is not possible with a raid4/5/6 journal device;208 it has to be deconfigured before requesting these.209 210 [journal_mode <mode>]211 This option sets the caching mode on journaled raid4/5/6 raid sets212 (see 'journal_dev <dev>' above) to 'writethrough' or 'writeback'.213 If 'writeback' is selected the journal device has to be resilient214 and must not suffer from the 'write hole' problem itself (e.g. use215 raid1 or raid10) to avoid a single point of failure.216 217<#raid_devs>: The number of devices composing the array.218 Each device consists of two entries. The first is the device219 containing the metadata (if any); the second is the one containing the220 data. A Maximum of 64 metadata/data device entries are supported221 up to target version 1.8.0.222 1.9.0 supports up to 253 which is enforced by the used MD kernel runtime.223 224 If a drive has failed or is missing at creation time, a '-' can be225 given for both the metadata and data drives for a given position.226 227 228Example Tables229--------------230 231::232 233 # RAID4 - 4 data drives, 1 parity (no metadata devices)234 # No metadata devices specified to hold superblock/bitmap info235 # Chunk size of 1MiB236 # (Lines separated for easy reading)237 238 0 1960893648 raid \239 raid4 1 2048 \240 5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81241 242 # RAID4 - 4 data drives, 1 parity (with metadata devices)243 # Chunk size of 1MiB, force RAID initialization,244 # min recovery rate at 20 kiB/sec/disk245 246 0 1960893648 raid \247 raid4 4 2048 sync min_recovery_rate 20 \248 5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82249 250 251Status Output252-------------253'dmsetup table' displays the table used to construct the mapping.254The optional parameters are always printed in the order listed255above with "sync" or "nosync" always output ahead of the other256arguments, regardless of the order used when originally loading the table.257Arguments that can be repeated are ordered by value.258 259 260'dmsetup status' yields information on the state and health of the array.261The output is as follows (normally a single line, but expanded here for262clarity)::263 264 1: <s> <l> raid \265 2: <raid_type> <#devices> <health_chars> \266 3: <sync_ratio> <sync_action> <mismatch_cnt>267 268Line 1 is the standard output produced by device-mapper.269 270Line 2 & 3 are produced by the raid target and are best explained by example::271 272 0 1960893648 raid raid4 5 AAAAA 2/490221568 init 0273 274Here we can see the RAID type is raid4, there are 5 devices - all of275which are 'A'live, and the array is 2/490221568 complete with its initial276recovery. Here is a fuller description of the individual fields:277 278 =============== =========================================================279 <raid_type> Same as the <raid_type> used to create the array.280 <health_chars> One char for each device, indicating:281 282 - 'A' = alive and in-sync283 - 'a' = alive but not in-sync284 - 'D' = dead/failed.285 <sync_ratio> The ratio indicating how much of the array has undergone286 the process described by 'sync_action'. If the287 'sync_action' is "check" or "repair", then the process288 of "resync" or "recover" can be considered complete.289 <sync_action> One of the following possible states:290 291 idle292 - No synchronization action is being performed.293 frozen294 - The current action has been halted.295 resync296 - Array is undergoing its initial synchronization297 or is resynchronizing after an unclean shutdown298 (possibly aided by a bitmap).299 recover300 - A device in the array is being rebuilt or301 replaced.302 check303 - A user-initiated full check of the array is304 being performed. All blocks are read and305 checked for consistency. The number of306 discrepancies found are recorded in307 <mismatch_cnt>. No changes are made to the308 array by this action.309 repair310 - The same as "check", but discrepancies are311 corrected.312 reshape313 - The array is undergoing a reshape.314 <mismatch_cnt> The number of discrepancies found between mirror copies315 in RAID1/10 or wrong parity values found in RAID4/5/6.316 This value is valid only after a "check" of the array317 is performed. A healthy array has a 'mismatch_cnt' of 0.318 <data_offset> The current data offset to the start of the user data on319 each component device of a raid set (see the respective320 raid parameter to support out-of-place reshaping).321 <journal_char> - 'A' - active write-through journal device.322 - 'a' - active write-back journal device.323 - 'D' - dead journal device.324 - '-' - no journal device.325 =============== =========================================================326 327 328Message Interface329-----------------330The dm-raid target will accept certain actions through the 'message' interface.331('man dmsetup' for more information on the message interface.) These actions332include:333 334 ========= ================================================335 "idle" Halt the current sync action.336 "frozen" Freeze the current sync action.337 "resync" Initiate/continue a resync.338 "recover" Initiate/continue a recover process.339 "check" Initiate a check (i.e. a "scrub") of the array.340 "repair" Initiate a repair of the array.341 ========= ================================================342 343 344Discard Support345---------------346The implementation of discard support among hardware vendors varies.347When a block is discarded, some storage devices will return zeroes when348the block is read. These devices set the 'discard_zeroes_data'349attribute. Other devices will return random data. Confusingly, some350devices that advertise 'discard_zeroes_data' will not reliably return351zeroes when discarded blocks are read! Since RAID 4/5/6 uses blocks352from a number of devices to calculate parity blocks and (for performance353reasons) relies on 'discard_zeroes_data' being reliable, it is important354that the devices be consistent. Blocks may be discarded in the middle355of a RAID 4/5/6 stripe and if subsequent read results are not356consistent, the parity blocks may be calculated differently at any time;357making the parity blocks useless for redundancy. It is important to358understand how your hardware behaves with discards if you are going to359enable discards with RAID 4/5/6.360 361Since the behavior of storage devices is unreliable in this respect,362even when reporting 'discard_zeroes_data', by default RAID 4/5/6363discard support is disabled -- this ensures data integrity at the364expense of losing some performance.365 366Storage devices that properly support 'discard_zeroes_data' are367increasingly whitelisted in the kernel and can thus be trusted.368 369For trusted devices, the following dm-raid module parameter can be set370to safely enable discard support for RAID 4/5/6:371 372 'devices_handle_discards_safely'373 374 375Version History376---------------377 378::379 380 1.0.0 Initial version. Support for RAID 4/5/6381 1.1.0 Added support for RAID 1382 1.2.0 Handle creation of arrays that contain failed devices.383 1.3.0 Added support for RAID 10384 1.3.1 Allow device replacement/rebuild for RAID 10385 1.3.2 Fix/improve redundancy checking for RAID10386 1.4.0 Non-functional change. Removes arg from mapping function.387 1.4.1 RAID10 fix redundancy validation checks (commit 55ebbb5).388 1.4.2 Add RAID10 "far" and "offset" algorithm support.389 1.5.0 Add message interface to allow manipulation of the sync_action.390 New status (STATUSTYPE_INFO) fields: sync_action and mismatch_cnt.391 1.5.1 Add ability to restore transiently failed devices on resume.392 1.5.2 'mismatch_cnt' is zero unless [last_]sync_action is "check".393 1.6.0 Add discard support (and devices_handle_discard_safely module param).394 1.7.0 Add support for MD RAID0 mappings.395 1.8.0 Explicitly check for compatible flags in the superblock metadata396 and reject to start the raid set if any are set by a newer397 target version, thus avoiding data corruption on a raid set398 with a reshape in progress.399 1.9.0 Add support for RAID level takeover/reshape/region size400 and set size reduction.401 1.9.1 Fix activation of existing RAID 4/10 mapped devices402 1.9.2 Don't emit '- -' on the status table line in case the constructor403 fails reading a superblock. Correctly emit 'maj:min1 maj:min2' and404 'D' on the status line. If '- -' is passed into the constructor, emit405 '- -' on the table line and '-' as the status line health character.406 1.10.0 Add support for raid4/5/6 journal device407 1.10.1 Fix data corruption on reshape request408 1.11.0 Fix table line argument order409 (wrong raid10_copies/raid10_format sequence)410 1.11.1 Add raid4/5/6 journal write-back support via journal_mode option411 1.12.1 Fix for MD deadlock between mddev_suspend() and md_write_start() available412 1.13.0 Fix dev_health status at end of "recover" (was 'a', now 'A')413 1.13.1 Fix deadlock caused by early md_stop_writes(). Also fix size an414 state races.415 1.13.2 Fix raid redundancy validation and avoid keeping raid set frozen416 1.14.0 Fix reshape race on small devices. Fix stripe adding reshape417 deadlock/potential data corruption. Update superblock when418 specific devices are requested via rebuild. Fix RAID leg419 rebuild errors.420 1.15.0 Fix size extensions not being synchronized in case of new MD bitmap421 pages allocated; also fix those not occurring after previous reductions422 1.15.1 Fix argument count and arguments for rebuild/write_mostly/journal_(dev|mode)423 on the status line.424