1Tools that manage md devices can be found at 2 http://www.kernel.org/pub/linux/utils/raid/ 3 4 5Boot time assembly of RAID arrays 6--------------------------------- 7 8You can boot with your md device with the following kernel command 9lines: 10 11for old raid arrays without persistent superblocks: 12 md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn 13 14for raid arrays with persistent superblocks 15 md=<md device no.>,dev0,dev1,...,devn 16or, to assemble a partitionable array: 17 md=d<md device no.>,dev0,dev1,...,devn 18 19md device no. = the number of the md device ... 20 0 means md0, 21 1 md1, 22 2 md2, 23 3 md3, 24 4 md4 25 26raid level = -1 linear mode 27 0 striped mode 28 other modes are only supported with persistent super blocks 29 30chunk size factor = (raid-0 and raid-1 only) 31 Set the chunk size as 4k << n. 32 33fault level = totally ignored 34 35dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1 36 37A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this: 38 39e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro 40 41 42Boot time autodetection of RAID arrays 43-------------------------------------- 44 45When md is compiled into the kernel (not as module), partitions of 46type 0xfd are scanned and automatically assembled into RAID arrays. 47This autodetection may be suppressed with the kernel parameter 48"raid=noautodetect". As of kernel 2.6.9, only drives with a type 0 49superblock can be autodetected and run at boot time. 50 51The kernel parameter "raid=partitionable" (or "raid=part") means 52that all auto-detected arrays are assembled as partitionable. 53 54Boot time assembly of degraded/dirty arrays 55------------------------------------------- 56 57If a raid5 or raid6 array is both dirty and degraded, it could have 58undetectable data corruption. This is because the fact that it is 59'dirty' means that the parity cannot be trusted, and the fact that it 60is degraded means that some datablocks are missing and cannot reliably 61be reconstructed (due to no parity). 62 63For this reason, md will normally refuse to start such an array. This 64requires the sysadmin to take action to explicitly start the array 65despite possible corruption. This is normally done with 66 mdadm --assemble --force .... 67 68This option is not really available if the array has the root 69filesystem on it. In order to support this booting from such an 70array, md supports a module parameter "start_dirty_degraded" which, 71when set to 1, bypassed the checks and will allows dirty degraded 72arrays to be started. 73 74So, to boot with a root filesystem of a dirty degraded raid[56], use 75 76 md-mod.start_dirty_degraded=1 77 78 79Superblock formats 80------------------ 81 82The md driver can support a variety of different superblock formats. 83Currently, it supports superblock formats "0.90.0" and the "md-1" format 84introduced in the 2.5 development series. 85 86The kernel will autodetect which format superblock is being used. 87 88Superblock format '0' is treated differently to others for legacy 89reasons - it is the original superblock format. 90 91 92General Rules - apply for all superblock formats 93------------------------------------------------ 94 95An array is 'created' by writing appropriate superblocks to all 96devices. 97 98It is 'assembled' by associating each of these devices with an 99particular md virtual device. Once it is completely assembled, it can 100be accessed. 101 102An array should be created by a user-space tool. This will write 103superblocks to all devices. It will usually mark the array as 104'unclean', or with some devices missing so that the kernel md driver 105can create appropriate redundancy (copying in raid1, parity 106calculation in raid4/5). 107 108When an array is assembled, it is first initialized with the 109SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor 110version number. The major version number selects which superblock 111format is to be used. The minor number might be used to tune handling 112of the format, such as suggesting where on each device to look for the 113superblock. 114 115Then each device is added using the ADD_NEW_DISK ioctl. This 116provides, in particular, a major and minor number identifying the 117device to add. 118 119The array is started with the RUN_ARRAY ioctl. 120 121Once started, new devices can be added. They should have an 122appropriate superblock written to them, and then be passed in with 123ADD_NEW_DISK. 124 125Devices that have failed or are not yet active can be detached from an 126array using HOT_REMOVE_DISK. 127 128 129Specific Rules that apply to format-0 super block arrays, and 130 arrays with no superblock (non-persistent). 131------------------------------------------------------------- 132 133An array can be 'created' by describing the array (level, chunksize 134etc) in a SET_ARRAY_INFO ioctl. This must have major_version==0 and 135raid_disks != 0. 136 137Then uninitialized devices can be added with ADD_NEW_DISK. The 138structure passed to ADD_NEW_DISK must specify the state of the device 139and its role in the array. 140 141Once started with RUN_ARRAY, uninitialized spares can be added with 142HOT_ADD_DISK. 143 144 145 146MD devices in sysfs 147------------------- 148md devices appear in sysfs (/sys) as regular block devices, 149e.g. 150 /sys/block/md0 151 152Each 'md' device will contain a subdirectory called 'md' which 153contains further md-specific information about the device. 154 155All md devices contain: 156 level 157 a text file indicating the 'raid level'. e.g. raid0, raid1, 158 raid5, linear, multipath, faulty. 159 If no raid level has been set yet (array is still being 160 assembled), the value will reflect whatever has been written 161 to it, which may be a name like the above, or may be a number 162 such as '0', '5', etc. 163 164 raid_disks 165 a text file with a simple number indicating the number of devices 166 in a fully functional array. If this is not yet known, the file 167 will be empty. If an array is being resized this will contain 168 the new number of devices. 169 Some raid levels allow this value to be set while the array is 170 active. This will reconfigure the array. Otherwise it can only 171 be set while assembling an array. 172 A change to this attribute will not be permitted if it would 173 reduce the size of the array. To reduce the number of drives 174 in an e.g. raid5, the array size must first be reduced by 175 setting the 'array_size' attribute. 176 177 chunk_size 178 This is the size in bytes for 'chunks' and is only relevant to 179 raid levels that involve striping (0,4,5,6,10). The address space 180 of the array is conceptually divided into chunks and consecutive 181 chunks are striped onto neighbouring devices. 182 The size should be at least PAGE_SIZE (4k) and should be a power 183 of 2. This can only be set while assembling an array 184 185 layout 186 The "layout" for the array for the particular level. This is 187 simply a number that is interpretted differently by different 188 levels. It can be written while assembling an array. 189 190 array_size 191 This can be used to artificially constrain the available space in 192 the array to be less than is actually available on the combined 193 devices. Writing a number (in Kilobytes) which is less than 194 the available size will set the size. Any reconfiguration of the 195 array (e.g. adding devices) will not cause the size to change. 196 Writing the word 'default' will cause the effective size of the 197 array to be whatever size is actually available based on 198 'level', 'chunk_size' and 'component_size'. 199 200 This can be used to reduce the size of the array before reducing 201 the number of devices in a raid4/5/6, or to support external 202 metadata formats which mandate such clipping. 203 204 reshape_position 205 This is either "none" or a sector number within the devices of 206 the array where "reshape" is up to. If this is set, the three 207 attributes mentioned above (raid_disks, chunk_size, layout) can 208 potentially have 2 values, an old and a new value. If these 209 values differ, reading the attribute returns 210 new (old) 211 and writing will effect the 'new' value, leaving the 'old' 212 unchanged. 213 214 component_size 215 For arrays with data redundancy (i.e. not raid0, linear, faulty, 216 multipath), all components must be the same size - or at least 217 there must a size that they all provide space for. This is a key 218 part or the geometry of the array. It is measured in sectors 219 and can be read from here. Writing to this value may resize 220 the array if the personality supports it (raid1, raid5, raid6), 221 and if the component drives are large enough. 222 223 metadata_version 224 This indicates the format that is being used to record metadata 225 about the array. It can be 0.90 (traditional format), 1.0, 1.1, 226 1.2 (newer format in varying locations) or "none" indicating that 227 the kernel isn't managing metadata at all. 228 Alternately it can be "external:" followed by a string which 229 is set by user-space. This indicates that metadata is managed 230 by a user-space program. Any device failure or other event that 231 requires a metadata update will cause array activity to be 232 suspended until the event is acknowledged. 233 234 resync_start 235 The point at which resync should start. If no resync is needed, 236 this will be a very large number (or 'none' since 2.6.30-rc1). At 237 array creation it will default to 0, though starting the array as 238 'clean' will set it much larger. 239 240 new_dev 241 This file can be written but not read. The value written should 242 be a block device number as major:minor. e.g. 8:0 243 This will cause that device to be attached to the array, if it is 244 available. It will then appear at md/dev-XXX (depending on the 245 name of the device) and further configuration is then possible. 246 247 safe_mode_delay 248 When an md array has seen no write requests for a certain period 249 of time, it will be marked as 'clean'. When another write 250 request arrives, the array is marked as 'dirty' before the write 251 commences. This is known as 'safe_mode'. 252 The 'certain period' is controlled by this file which stores the 253 period as a number of seconds. The default is 200msec (0.200). 254 Writing a value of 0 disables safemode. 255 256 array_state 257 This file contains a single word which describes the current 258 state of the array. In many cases, the state can be set by 259 writing the word for the desired state, however some states 260 cannot be explicitly set, and some transitions are not allowed. 261 262 Select/poll works on this file. All changes except between 263 active_idle and active (which can be frequent and are not 264 very interesting) are notified. active->active_idle is 265 reported if the metadata is externally managed. 266 267 clear 268 No devices, no size, no level 269 Writing is equivalent to STOP_ARRAY ioctl 270 inactive 271 May have some settings, but array is not active 272 all IO results in error 273 When written, doesn't tear down array, but just stops it 274 suspended (not supported yet) 275 All IO requests will block. The array can be reconfigured. 276 Writing this, if accepted, will block until array is quiessent 277 readonly 278 no resync can happen. no superblocks get written. 279 write requests fail 280 read-auto 281 like readonly, but behaves like 'clean' on a write request. 282 283 clean - no pending writes, but otherwise active. 284 When written to inactive array, starts without resync 285 If a write request arrives then 286 if metadata is known, mark 'dirty' and switch to 'active'. 287 if not known, block and switch to write-pending 288 If written to an active array that has pending writes, then fails. 289 active 290 fully active: IO and resync can be happening. 291 When written to inactive array, starts with resync 292 293 write-pending 294 clean, but writes are blocked waiting for 'active' to be written. 295 296 active-idle 297 like active, but no writes have been seen for a while (safe_mode_delay). 298 299 bitmap/location 300 This indicates where the write-intent bitmap for the array is 301 stored. 302 It can be one of "none", "file" or "[+-]N". 303 "file" may later be extended to "file:/file/name" 304 "[+-]N" means that many sectors from the start of the metadata. 305 This is replicated on all devices. For arrays with externally 306 managed metadata, the offset is from the beginning of the 307 device. 308 bitmap/chunksize 309 The size, in bytes, of the chunk which will be represented by a 310 single bit. For RAID456, it is a portion of an individual 311 device. For RAID10, it is a portion of the array. For RAID1, it 312 is both (they come to the same thing). 313 bitmap/time_base 314 The time, in seconds, between looking for bits in the bitmap to 315 be cleared. In the current implementation, a bit will be cleared 316 between 2 and 3 times "time_base" after all the covered blocks 317 are known to be in-sync. 318 bitmap/backlog 319 When write-mostly devices are active in a RAID1, write requests 320 to those devices proceed in the background - the filesystem (or 321 other user of the device) does not have to wait for them. 322 'backlog' sets a limit on the number of concurrent background 323 writes. If there are more than this, new writes will by 324 synchronous. 325 bitmap/metadata 326 This can be either 'internal' or 'external'. 327 'internal' is the default and means the metadata for the bitmap 328 is stored in the first 256 bytes of the allocated space and is 329 managed by the md module. 330 'external' means that bitmap metadata is managed externally to 331 the kernel (i.e. by some userspace program) 332 bitmap/can_clear 333 This is either 'true' or 'false'. If 'true', then bits in the 334 bitmap will be cleared when the corresponding blocks are thought 335 to be in-sync. If 'false', bits will never be cleared. 336 This is automatically set to 'false' if a write happens on a 337 degraded array, or if the array becomes degraded during a write. 338 When metadata is managed externally, it should be set to true 339 once the array becomes non-degraded, and this fact has been 340 recorded in the metadata. 341 342 343 344 345As component devices are added to an md array, they appear in the 'md' 346directory as new directories named 347 dev-XXX 348where XXX is a name that the kernel knows for the device, e.g. hdb1. 349Each directory contains: 350 351 block 352 a symlink to the block device in /sys/block, e.g. 353 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 354 355 super 356 A file containing an image of the superblock read from, or 357 written to, that device. 358 359 state 360 A file recording the current state of the device in the array 361 which can be a comma separated list of 362 faulty - device has been kicked from active use due to 363 a detected fault, or it has unacknowledged bad 364 blocks 365 in_sync - device is a fully in-sync member of the array 366 writemostly - device will only be subject to read 367 requests if there are no other options. 368 This applies only to raid1 arrays. 369 blocked - device has failed, and the failure hasn't been 370 acknowledged yet by the metadata handler. 371 Writes that would write to this device if 372 it were not faulty are blocked. 373 spare - device is working, but not a full member. 374 This includes spares that are in the process 375 of being recovered to 376 write_error - device has ever seen a write error. 377 want_replacement - device is (mostly) working but probably 378 should be replaced, either due to errors or 379 due to user request. 380 replacement - device is a replacement for another active 381 device with same raid_disk. 382 383 384 This list may grow in future. 385 This can be written to. 386 Writing "faulty" simulates a failure on the device. 387 Writing "remove" removes the device from the array. 388 Writing "writemostly" sets the writemostly flag. 389 Writing "-writemostly" clears the writemostly flag. 390 Writing "blocked" sets the "blocked" flag. 391 Writing "-blocked" clears the "blocked" flags and allows writes 392 to complete and possibly simulates an error. 393 Writing "in_sync" sets the in_sync flag. 394 Writing "write_error" sets writeerrorseen flag. 395 Writing "-write_error" clears writeerrorseen flag. 396 Writing "want_replacement" is allowed at any time except to a 397 replacement device or a spare. It sets the flag. 398 Writing "-want_replacement" is allowed at any time. It clears 399 the flag. 400 Writing "replacement" or "-replacement" is only allowed before 401 starting the array. It sets or clears the flag. 402 403 404 This file responds to select/poll. Any change to 'faulty' 405 or 'blocked' causes an event. 406 407 errors 408 An approximate count of read errors that have been detected on 409 this device but have not caused the device to be evicted from 410 the array (either because they were corrected or because they 411 happened while the array was read-only). When using version-1 412 metadata, this value persists across restarts of the array. 413 414 This value can be written while assembling an array thus 415 providing an ongoing count for arrays with metadata managed by 416 userspace. 417 418 slot 419 This gives the role that the device has in the array. It will 420 either be 'none' if the device is not active in the array 421 (i.e. is a spare or has failed) or an integer less than the 422 'raid_disks' number for the array indicating which position 423 it currently fills. This can only be set while assembling an 424 array. A device for which this is set is assumed to be working. 425 426 offset 427 This gives the location in the device (in sectors from the 428 start) where data from the array will be stored. Any part of 429 the device before this offset is not touched, unless it is 430 used for storing metadata (Formats 1.1 and 1.2). 431 432 size 433 The amount of the device, after the offset, that can be used 434 for storage of data. This will normally be the same as the 435 component_size. This can be written while assembling an 436 array. If a value less than the current component_size is 437 written, it will be rejected. 438 439 recovery_start 440 When the device is not 'in_sync', this records the number of 441 sectors from the start of the device which are known to be 442 correct. This is normally zero, but during a recovery 443 operation it will steadily increase, and if the recovery is 444 interrupted, restoring this value can cause recovery to 445 avoid repeating the earlier blocks. With v1.x metadata, this 446 value is saved and restored automatically. 447 448 This can be set whenever the device is not an active member of 449 the array, either before the array is activated, or before 450 the 'slot' is set. 451 452 Setting this to 'none' is equivalent to setting 'in_sync'. 453 Setting to any other value also clears the 'in_sync' flag. 454 455 bad_blocks 456 This gives the list of all known bad blocks in the form of 457 start address and length (in sectors respectively). If output 458 is too big to fit in a page, it will be truncated. Writing 459 "sector length" to this file adds new acknowledged (i.e. 460 recorded to disk safely) bad blocks. 461 462 unacknowledged_bad_blocks 463 This gives the list of known-but-not-yet-saved-to-disk bad 464 blocks in the same form of 'bad_blocks'. If output is too big 465 to fit in a page, it will be truncated. Writing to this file 466 adds bad blocks without acknowledging them. This is largely 467 for testing. 468 469 470 471An active md device will also contain an entry for each active device 472in the array. These are named 473 474 rdNN 475 476where 'NN' is the position in the array, starting from 0. 477So for a 3 drive array there will be rd0, rd1, rd2. 478These are symbolic links to the appropriate 'dev-XXX' entry. 479Thus, for example, 480 cat /sys/block/md*/md/rd*/state 481will show 'in_sync' on every line. 482 483 484 485Active md devices for levels that support data redundancy (1,4,5,6,10) 486also have 487 488 sync_action 489 a text file that can be used to monitor and control the rebuild 490 process. It contains one word which can be one of: 491 resync - redundancy is being recalculated after unclean 492 shutdown or creation 493 recover - a hot spare is being built to replace a 494 failed/missing device 495 idle - nothing is happening 496 check - A full check of redundancy was requested and is 497 happening. This reads all blocks and checks 498 them. A repair may also happen for some raid 499 levels. 500 repair - A full check and repair is happening. This is 501 similar to 'resync', but was requested by the 502 user, and the write-intent bitmap is NOT used to 503 optimise the process. 504 505 This file is writable, and each of the strings that could be 506 read are meaningful for writing. 507 508 'idle' will stop an active resync/recovery etc. There is no 509 guarantee that another resync/recovery may not be automatically 510 started again, though some event will be needed to trigger 511 this. 512 'resync' or 'recovery' can be used to restart the 513 corresponding operation if it was stopped with 'idle'. 514 'check' and 'repair' will start the appropriate process 515 providing the current state is 'idle'. 516 517 This file responds to select/poll. Any important change in the value 518 triggers a poll event. Sometimes the value will briefly be 519 "recover" if a recovery seems to be needed, but cannot be 520 achieved. In that case, the transition to "recover" isn't 521 notified, but the transition away is. 522 523 degraded 524 This contains a count of the number of devices by which the 525 arrays is degraded. So an optimal array will show '0'. A 526 single failed/missing drive will show '1', etc. 527 This file responds to select/poll, any increase or decrease 528 in the count of missing devices will trigger an event. 529 530 mismatch_count 531 When performing 'check' and 'repair', and possibly when 532 performing 'resync', md will count the number of errors that are 533 found. The count in 'mismatch_cnt' is the number of sectors 534 that were re-written, or (for 'check') would have been 535 re-written. As most raid levels work in units of pages rather 536 than sectors, this may be larger than the number of actual errors 537 by a factor of the number of sectors in a page. 538 539 bitmap_set_bits 540 If the array has a write-intent bitmap, then writing to this 541 attribute can set bits in the bitmap, indicating that a resync 542 would need to check the corresponding blocks. Either individual 543 numbers or start-end pairs can be written. Multiple numbers 544 can be separated by a space. 545 Note that the numbers are 'bit' numbers, not 'block' numbers. 546 They should be scaled by the bitmap_chunksize. 547 548 sync_speed_min 549 sync_speed_max 550 This are similar to /proc/sys/dev/raid/speed_limit_{min,max} 551 however they only apply to the particular array. 552 If no value has been written to these, of if the word 'system' 553 is written, then the system-wide value is used. If a value, 554 in kibibytes-per-second is written, then it is used. 555 When the files are read, they show the currently active value 556 followed by "(local)" or "(system)" depending on whether it is 557 a locally set or system-wide value. 558 559 sync_completed 560 This shows the number of sectors that have been completed of 561 whatever the current sync_action is, followed by the number of 562 sectors in total that could need to be processed. The two 563 numbers are separated by a '/' thus effectively showing one 564 value, a fraction of the process that is complete. 565 A 'select' on this attribute will return when resync completes, 566 when it reaches the current sync_max (below) and possibly at 567 other times. 568 569 sync_speed 570 This shows the current actual speed, in K/sec, of the current 571 sync_action. It is averaged over the last 30 seconds. 572 573 suspend_lo 574 suspend_hi 575 The two values, given as numbers of sectors, indicate a range 576 within the array where IO will be blocked. This is currently 577 only supported for raid4/5/6. 578 579 sync_min 580 sync_max 581 The two values, given as numbers of sectors, indicate a range 582 within the array where 'check'/'repair' will operate. Must be 583 a multiple of chunk_size. When it reaches "sync_max" it will 584 pause, rather than complete. 585 You can use 'select' or 'poll' on "sync_completed" to wait for 586 that number to reach sync_max. Then you can either increase 587 "sync_max", or can write 'idle' to "sync_action". 588 589 The value of 'max' for "sync_max" effectively disables the limit. 590 When a resync is active, the value can only ever be increased, 591 never decreased. 592 The value of '0' is the minimum for "sync_min". 593 594 595 596Each active md device may also have attributes specific to the 597personality module that manages it. 598These are specific to the implementation of the module and could 599change substantially if the implementation changes. 600 601These currently include 602 603 stripe_cache_size (currently raid5 only) 604 number of entries in the stripe cache. This is writable, but 605 there are upper and lower limits (32768, 16). Default is 128. 606 strip_cache_active (currently raid5 only) 607 number of active entries in the stripe cache 608 preread_bypass_threshold (currently raid5 only) 609 number of times a stripe requiring preread will be bypassed by 610 a stripe that does not require preread. For fairness defaults 611 to 1. Setting this to 0 disables bypass accounting and 612 requires preread stripes to wait until all full-width stripe- 613 writes are complete. Valid values are 0 to stripe_cache_size. 614