1RAID arrays 2=========== 3 4Boot time assembly of RAID arrays 5--------------------------------- 6 7Tools that manage md devices can be found at 8 http://www.kernel.org/pub/linux/utils/raid/ 9 10 11You can boot with your md device with the following kernel command 12lines: 13 14for old raid arrays without persistent superblocks:: 15 16 md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn 17 18for raid arrays with persistent superblocks:: 19 20 md=<md device no.>,dev0,dev1,...,devn 21 22or, to assemble a partitionable array:: 23 24 md=d<md device no.>,dev0,dev1,...,devn 25 26``md device no.`` 27+++++++++++++++++ 28 29The number of the md device 30 31================= ========= 32``md device no.`` device 33================= ========= 34 0 md0 35 1 md1 36 2 md2 37 3 md3 38 4 md4 39================= ========= 40 41``raid level`` 42++++++++++++++ 43 44level of the RAID array 45 46=============== ============= 47``raid level`` level 48=============== ============= 49-1 linear mode 500 striped mode 51=============== ============= 52 53other modes are only supported with persistent super blocks 54 55``chunk size factor`` 56+++++++++++++++++++++ 57 58(raid-0 and raid-1 only) 59 60Set the chunk size as 4k << n. 61 62``fault level`` 63+++++++++++++++ 64 65Totally ignored 66 67``dev0`` to ``devn`` 68++++++++++++++++++++ 69 70e.g. ``/dev/hda1``, ``/dev/hdc1``, ``/dev/sda1``, ``/dev/sdb1`` 71 72A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this:: 73 74 e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro 75 76 77Boot time autodetection of RAID arrays 78-------------------------------------- 79 80When md is compiled into the kernel (not as module), partitions of 81type 0xfd are scanned and automatically assembled into RAID arrays. 82This autodetection may be suppressed with the kernel parameter 83``raid=noautodetect``. As of kernel 2.6.9, only drives with a type 0 84superblock can be autodetected and run at boot time. 85 86The kernel parameter ``raid=partitionable`` (or ``raid=part``) means 87that all auto-detected arrays are assembled as partitionable. 88 89Boot time assembly of degraded/dirty arrays 90------------------------------------------- 91 92If a raid5 or raid6 array is both dirty and degraded, it could have 93undetectable data corruption. This is because the fact that it is 94``dirty`` means that the parity cannot be trusted, and the fact that it 95is degraded means that some datablocks are missing and cannot reliably 96be reconstructed (due to no parity). 97 98For this reason, md will normally refuse to start such an array. This 99requires the sysadmin to take action to explicitly start the array 100despite possible corruption. This is normally done with:: 101 102 mdadm --assemble --force .... 103 104This option is not really available if the array has the root 105filesystem on it. In order to support this booting from such an 106array, md supports a module parameter ``start_dirty_degraded`` which, 107when set to 1, bypassed the checks and will allows dirty degraded 108arrays to be started. 109 110So, to boot with a root filesystem of a dirty degraded raid 5 or 6, use:: 111 112 md-mod.start_dirty_degraded=1 113 114 115Superblock formats 116------------------ 117 118The md driver can support a variety of different superblock formats. 119Currently, it supports superblock formats ``0.90.0`` and the ``md-1`` format 120introduced in the 2.5 development series. 121 122The kernel will autodetect which format superblock is being used. 123 124Superblock format ``0`` is treated differently to others for legacy 125reasons - it is the original superblock format. 126 127 128General Rules - apply for all superblock formats 129------------------------------------------------ 130 131An array is ``created`` by writing appropriate superblocks to all 132devices. 133 134It is ``assembled`` by associating each of these devices with an 135particular md virtual device. Once it is completely assembled, it can 136be accessed. 137 138An array should be created by a user-space tool. This will write 139superblocks to all devices. It will usually mark the array as 140``unclean``, or with some devices missing so that the kernel md driver 141can create appropriate redundancy (copying in raid 1, parity 142calculation in raid 4/5). 143 144When an array is assembled, it is first initialized with the 145SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor 146version number. The major version number selects which superblock 147format is to be used. The minor number might be used to tune handling 148of the format, such as suggesting where on each device to look for the 149superblock. 150 151Then each device is added using the ADD_NEW_DISK ioctl. This 152provides, in particular, a major and minor number identifying the 153device to add. 154 155The array is started with the RUN_ARRAY ioctl. 156 157Once started, new devices can be added. They should have an 158appropriate superblock written to them, and then be passed in with 159ADD_NEW_DISK. 160 161Devices that have failed or are not yet active can be detached from an 162array using HOT_REMOVE_DISK. 163 164 165Specific Rules that apply to format-0 super block arrays, and arrays with no superblock (non-persistent) 166-------------------------------------------------------------------------------------------------------- 167 168An array can be ``created`` by describing the array (level, chunksize 169etc) in a SET_ARRAY_INFO ioctl. This must have ``major_version==0`` and 170``raid_disks != 0``. 171 172Then uninitialized devices can be added with ADD_NEW_DISK. The 173structure passed to ADD_NEW_DISK must specify the state of the device 174and its role in the array. 175 176Once started with RUN_ARRAY, uninitialized spares can be added with 177HOT_ADD_DISK. 178 179 180MD devices in sysfs 181------------------- 182 183md devices appear in sysfs (``/sys``) as regular block devices, 184e.g.:: 185 186 /sys/block/md0 187 188Each ``md`` device will contain a subdirectory called ``md`` which 189contains further md-specific information about the device. 190 191All md devices contain: 192 193 level 194 a text file indicating the ``raid level``. e.g. raid0, raid1, 195 raid5, linear, multipath, faulty. 196 If no raid level has been set yet (array is still being 197 assembled), the value will reflect whatever has been written 198 to it, which may be a name like the above, or may be a number 199 such as ``0``, ``5``, etc. 200 201 raid_disks 202 a text file with a simple number indicating the number of devices 203 in a fully functional array. If this is not yet known, the file 204 will be empty. If an array is being resized this will contain 205 the new number of devices. 206 Some raid levels allow this value to be set while the array is 207 active. This will reconfigure the array. Otherwise it can only 208 be set while assembling an array. 209 A change to this attribute will not be permitted if it would 210 reduce the size of the array. To reduce the number of drives 211 in an e.g. raid5, the array size must first be reduced by 212 setting the ``array_size`` attribute. 213 214 chunk_size 215 This is the size in bytes for ``chunks`` and is only relevant to 216 raid levels that involve striping (0,4,5,6,10). The address space 217 of the array is conceptually divided into chunks and consecutive 218 chunks are striped onto neighbouring devices. 219 The size should be at least PAGE_SIZE (4k) and should be a power 220 of 2. This can only be set while assembling an array 221 222 layout 223 The ``layout`` for the array for the particular level. This is 224 simply a number that is interpretted differently by different 225 levels. It can be written while assembling an array. 226 227 array_size 228 This can be used to artificially constrain the available space in 229 the array to be less than is actually available on the combined 230 devices. Writing a number (in Kilobytes) which is less than 231 the available size will set the size. Any reconfiguration of the 232 array (e.g. adding devices) will not cause the size to change. 233 Writing the word ``default`` will cause the effective size of the 234 array to be whatever size is actually available based on 235 ``level``, ``chunk_size`` and ``component_size``. 236 237 This can be used to reduce the size of the array before reducing 238 the number of devices in a raid4/5/6, or to support external 239 metadata formats which mandate such clipping. 240 241 reshape_position 242 This is either ``none`` or a sector number within the devices of 243 the array where ``reshape`` is up to. If this is set, the three 244 attributes mentioned above (raid_disks, chunk_size, layout) can 245 potentially have 2 values, an old and a new value. If these 246 values differ, reading the attribute returns:: 247 248 new (old) 249 250 and writing will effect the ``new`` value, leaving the ``old`` 251 unchanged. 252 253 component_size 254 For arrays with data redundancy (i.e. not raid0, linear, faulty, 255 multipath), all components must be the same size - or at least 256 there must a size that they all provide space for. This is a key 257 part or the geometry of the array. It is measured in sectors 258 and can be read from here. Writing to this value may resize 259 the array if the personality supports it (raid1, raid5, raid6), 260 and if the component drives are large enough. 261 262 metadata_version 263 This indicates the format that is being used to record metadata 264 about the array. It can be 0.90 (traditional format), 1.0, 1.1, 265 1.2 (newer format in varying locations) or ``none`` indicating that 266 the kernel isn't managing metadata at all. 267 Alternately it can be ``external:`` followed by a string which 268 is set by user-space. This indicates that metadata is managed 269 by a user-space program. Any device failure or other event that 270 requires a metadata update will cause array activity to be 271 suspended until the event is acknowledged. 272 273 resync_start 274 The point at which resync should start. If no resync is needed, 275 this will be a very large number (or ``none`` since 2.6.30-rc1). At 276 array creation it will default to 0, though starting the array as 277 ``clean`` will set it much larger. 278 279 new_dev 280 This file can be written but not read. The value written should 281 be a block device number as major:minor. e.g. 8:0 282 This will cause that device to be attached to the array, if it is 283 available. It will then appear at md/dev-XXX (depending on the 284 name of the device) and further configuration is then possible. 285 286 safe_mode_delay 287 When an md array has seen no write requests for a certain period 288 of time, it will be marked as ``clean``. When another write 289 request arrives, the array is marked as ``dirty`` before the write 290 commences. This is known as ``safe_mode``. 291 The ``certain period`` is controlled by this file which stores the 292 period as a number of seconds. The default is 200msec (0.200). 293 Writing a value of 0 disables safemode. 294 295 array_state 296 This file contains a single word which describes the current 297 state of the array. In many cases, the state can be set by 298 writing the word for the desired state, however some states 299 cannot be explicitly set, and some transitions are not allowed. 300 301 Select/poll works on this file. All changes except between 302 Active_idle and active (which can be frequent and are not 303 very interesting) are notified. active->active_idle is 304 reported if the metadata is externally managed. 305 306 clear 307 No devices, no size, no level 308 309 Writing is equivalent to STOP_ARRAY ioctl 310 311 inactive 312 May have some settings, but array is not active 313 all IO results in error 314 315 When written, doesn't tear down array, but just stops it 316 317 suspended (not supported yet) 318 All IO requests will block. The array can be reconfigured. 319 320 Writing this, if accepted, will block until array is quiessent 321 322 readonly 323 no resync can happen. no superblocks get written. 324 325 Write requests fail 326 327 read-auto 328 like readonly, but behaves like ``clean`` on a write request. 329 330 clean 331 no pending writes, but otherwise active. 332 333 When written to inactive array, starts without resync 334 335 If a write request arrives then 336 if metadata is known, mark ``dirty`` and switch to ``active``. 337 if not known, block and switch to write-pending 338 339 If written to an active array that has pending writes, then fails. 340 active 341 fully active: IO and resync can be happening. 342 When written to inactive array, starts with resync 343 344 write-pending 345 clean, but writes are blocked waiting for ``active`` to be written. 346 347 active-idle 348 like active, but no writes have been seen for a while (safe_mode_delay). 349 350 bitmap/location 351 This indicates where the write-intent bitmap for the array is 352 stored. 353 354 It can be one of ``none``, ``file`` or ``[+-]N``. 355 ``file`` may later be extended to ``file:/file/name`` 356 ``[+-]N`` means that many sectors from the start of the metadata. 357 358 This is replicated on all devices. For arrays with externally 359 managed metadata, the offset is from the beginning of the 360 device. 361 362 bitmap/chunksize 363 The size, in bytes, of the chunk which will be represented by a 364 single bit. For RAID456, it is a portion of an individual 365 device. For RAID10, it is a portion of the array. For RAID1, it 366 is both (they come to the same thing). 367 368 bitmap/time_base 369 The time, in seconds, between looking for bits in the bitmap to 370 be cleared. In the current implementation, a bit will be cleared 371 between 2 and 3 times ``time_base`` after all the covered blocks 372 are known to be in-sync. 373 374 bitmap/backlog 375 When write-mostly devices are active in a RAID1, write requests 376 to those devices proceed in the background - the filesystem (or 377 other user of the device) does not have to wait for them. 378 ``backlog`` sets a limit on the number of concurrent background 379 writes. If there are more than this, new writes will by 380 synchronous. 381 382 bitmap/metadata 383 This can be either ``internal`` or ``external``. 384 385 ``internal`` 386 is the default and means the metadata for the bitmap 387 is stored in the first 256 bytes of the allocated space and is 388 managed by the md module. 389 390 ``external`` 391 means that bitmap metadata is managed externally to 392 the kernel (i.e. by some userspace program) 393 394 bitmap/can_clear 395 This is either ``true`` or ``false``. If ``true``, then bits in the 396 bitmap will be cleared when the corresponding blocks are thought 397 to be in-sync. If ``false``, bits will never be cleared. 398 This is automatically set to ``false`` if a write happens on a 399 degraded array, or if the array becomes degraded during a write. 400 When metadata is managed externally, it should be set to true 401 once the array becomes non-degraded, and this fact has been 402 recorded in the metadata. 403 404 405 406 407As component devices are added to an md array, they appear in the ``md`` 408directory as new directories named:: 409 410 dev-XXX 411 412where ``XXX`` is a name that the kernel knows for the device, e.g. hdb1. 413Each directory contains: 414 415 block 416 a symlink to the block device in /sys/block, e.g.:: 417 418 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 419 420 super 421 A file containing an image of the superblock read from, or 422 written to, that device. 423 424 state 425 A file recording the current state of the device in the array 426 which can be a comma separated list of: 427 428 faulty 429 device has been kicked from active use due to 430 a detected fault, or it has unacknowledged bad 431 blocks 432 433 in_sync 434 device is a fully in-sync member of the array 435 436 writemostly 437 device will only be subject to read 438 requests if there are no other options. 439 440 This applies only to raid1 arrays. 441 442 blocked 443 device has failed, and the failure hasn't been 444 acknowledged yet by the metadata handler. 445 446 Writes that would write to this device if 447 it were not faulty are blocked. 448 449 spare 450 device is working, but not a full member. 451 452 This includes spares that are in the process 453 of being recovered to 454 455 write_error 456 device has ever seen a write error. 457 458 want_replacement 459 device is (mostly) working but probably 460 should be replaced, either due to errors or 461 due to user request. 462 463 replacement 464 device is a replacement for another active 465 device with same raid_disk. 466 467 468 This list may grow in future. 469 470 This can be written to. 471 472 Writing ``faulty`` simulates a failure on the device. 473 474 Writing ``remove`` removes the device from the array. 475 476 Writing ``writemostly`` sets the writemostly flag. 477 478 Writing ``-writemostly`` clears the writemostly flag. 479 480 Writing ``blocked`` sets the ``blocked`` flag. 481 482 Writing ``-blocked`` clears the ``blocked`` flags and allows writes 483 to complete and possibly simulates an error. 484 485 Writing ``in_sync`` sets the in_sync flag. 486 487 Writing ``write_error`` sets writeerrorseen flag. 488 489 Writing ``-write_error`` clears writeerrorseen flag. 490 491 Writing ``want_replacement`` is allowed at any time except to a 492 replacement device or a spare. It sets the flag. 493 494 Writing ``-want_replacement`` is allowed at any time. It clears 495 the flag. 496 497 Writing ``replacement`` or ``-replacement`` is only allowed before 498 starting the array. It sets or clears the flag. 499 500 501 This file responds to select/poll. Any change to ``faulty`` 502 or ``blocked`` causes an event. 503 504 errors 505 An approximate count of read errors that have been detected on 506 this device but have not caused the device to be evicted from 507 the array (either because they were corrected or because they 508 happened while the array was read-only). When using version-1 509 metadata, this value persists across restarts of the array. 510 511 This value can be written while assembling an array thus 512 providing an ongoing count for arrays with metadata managed by 513 userspace. 514 515 slot 516 This gives the role that the device has in the array. It will 517 either be ``none`` if the device is not active in the array 518 (i.e. is a spare or has failed) or an integer less than the 519 ``raid_disks`` number for the array indicating which position 520 it currently fills. This can only be set while assembling an 521 array. A device for which this is set is assumed to be working. 522 523 offset 524 This gives the location in the device (in sectors from the 525 start) where data from the array will be stored. Any part of 526 the device before this offset is not touched, unless it is 527 used for storing metadata (Formats 1.1 and 1.2). 528 529 size 530 The amount of the device, after the offset, that can be used 531 for storage of data. This will normally be the same as the 532 component_size. This can be written while assembling an 533 array. If a value less than the current component_size is 534 written, it will be rejected. 535 536 recovery_start 537 When the device is not ``in_sync``, this records the number of 538 sectors from the start of the device which are known to be 539 correct. This is normally zero, but during a recovery 540 operation it will steadily increase, and if the recovery is 541 interrupted, restoring this value can cause recovery to 542 avoid repeating the earlier blocks. With v1.x metadata, this 543 value is saved and restored automatically. 544 545 This can be set whenever the device is not an active member of 546 the array, either before the array is activated, or before 547 the ``slot`` is set. 548 549 Setting this to ``none`` is equivalent to setting ``in_sync``. 550 Setting to any other value also clears the ``in_sync`` flag. 551 552 bad_blocks 553 This gives the list of all known bad blocks in the form of 554 start address and length (in sectors respectively). If output 555 is too big to fit in a page, it will be truncated. Writing 556 ``sector length`` to this file adds new acknowledged (i.e. 557 recorded to disk safely) bad blocks. 558 559 unacknowledged_bad_blocks 560 This gives the list of known-but-not-yet-saved-to-disk bad 561 blocks in the same form of ``bad_blocks``. If output is too big 562 to fit in a page, it will be truncated. Writing to this file 563 adds bad blocks without acknowledging them. This is largely 564 for testing. 565 566 567 568An active md device will also contain an entry for each active device 569in the array. These are named:: 570 571 rdNN 572 573where ``NN`` is the position in the array, starting from 0. 574So for a 3 drive array there will be rd0, rd1, rd2. 575These are symbolic links to the appropriate ``dev-XXX`` entry. 576Thus, for example:: 577 578 cat /sys/block/md*/md/rd*/state 579 580will show ``in_sync`` on every line. 581 582 583 584Active md devices for levels that support data redundancy (1,4,5,6,10) 585also have 586 587 sync_action 588 a text file that can be used to monitor and control the rebuild 589 process. It contains one word which can be one of: 590 591 resync 592 redundancy is being recalculated after unclean 593 shutdown or creation 594 595 recover 596 a hot spare is being built to replace a 597 failed/missing device 598 599 idle 600 nothing is happening 601 check 602 A full check of redundancy was requested and is 603 happening. This reads all blocks and checks 604 them. A repair may also happen for some raid 605 levels. 606 607 repair 608 A full check and repair is happening. This is 609 similar to ``resync``, but was requested by the 610 user, and the write-intent bitmap is NOT used to 611 optimise the process. 612 613 This file is writable, and each of the strings that could be 614 read are meaningful for writing. 615 616 ``idle`` will stop an active resync/recovery etc. There is no 617 guarantee that another resync/recovery may not be automatically 618 started again, though some event will be needed to trigger 619 this. 620 621 ``resync`` or ``recovery`` can be used to restart the 622 corresponding operation if it was stopped with ``idle``. 623 624 ``check`` and ``repair`` will start the appropriate process 625 providing the current state is ``idle``. 626 627 This file responds to select/poll. Any important change in the value 628 triggers a poll event. Sometimes the value will briefly be 629 ``recover`` if a recovery seems to be needed, but cannot be 630 achieved. In that case, the transition to ``recover`` isn't 631 notified, but the transition away is. 632 633 degraded 634 This contains a count of the number of devices by which the 635 arrays is degraded. So an optimal array will show ``0``. A 636 single failed/missing drive will show ``1``, etc. 637 638 This file responds to select/poll, any increase or decrease 639 in the count of missing devices will trigger an event. 640 641 mismatch_count 642 When performing ``check`` and ``repair``, and possibly when 643 performing ``resync``, md will count the number of errors that are 644 found. The count in ``mismatch_cnt`` is the number of sectors 645 that were re-written, or (for ``check``) would have been 646 re-written. As most raid levels work in units of pages rather 647 than sectors, this may be larger than the number of actual errors 648 by a factor of the number of sectors in a page. 649 650 bitmap_set_bits 651 If the array has a write-intent bitmap, then writing to this 652 attribute can set bits in the bitmap, indicating that a resync 653 would need to check the corresponding blocks. Either individual 654 numbers or start-end pairs can be written. Multiple numbers 655 can be separated by a space. 656 657 Note that the numbers are ``bit`` numbers, not ``block`` numbers. 658 They should be scaled by the bitmap_chunksize. 659 660 sync_speed_min, sync_speed_max 661 This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}`` 662 however they only apply to the particular array. 663 664 If no value has been written to these, or if the word ``system`` 665 is written, then the system-wide value is used. If a value, 666 in kibibytes-per-second is written, then it is used. 667 668 When the files are read, they show the currently active value 669 followed by ``(local)`` or ``(system)`` depending on whether it is 670 a locally set or system-wide value. 671 672 sync_completed 673 This shows the number of sectors that have been completed of 674 whatever the current sync_action is, followed by the number of 675 sectors in total that could need to be processed. The two 676 numbers are separated by a ``/`` thus effectively showing one 677 value, a fraction of the process that is complete. 678 679 A ``select`` on this attribute will return when resync completes, 680 when it reaches the current sync_max (below) and possibly at 681 other times. 682 683 sync_speed 684 This shows the current actual speed, in K/sec, of the current 685 sync_action. It is averaged over the last 30 seconds. 686 687 suspend_lo, suspend_hi 688 The two values, given as numbers of sectors, indicate a range 689 within the array where IO will be blocked. This is currently 690 only supported for raid4/5/6. 691 692 sync_min, sync_max 693 The two values, given as numbers of sectors, indicate a range 694 within the array where ``check``/``repair`` will operate. Must be 695 a multiple of chunk_size. When it reaches ``sync_max`` it will 696 pause, rather than complete. 697 You can use ``select`` or ``poll`` on ``sync_completed`` to wait for 698 that number to reach sync_max. Then you can either increase 699 ``sync_max``, or can write ``idle`` to ``sync_action``. 700 701 The value of ``max`` for ``sync_max`` effectively disables the limit. 702 When a resync is active, the value can only ever be increased, 703 never decreased. 704 The value of ``0`` is the minimum for ``sync_min``. 705 706 707 708Each active md device may also have attributes specific to the 709personality module that manages it. 710These are specific to the implementation of the module and could 711change substantially if the implementation changes. 712 713These currently include: 714 715 stripe_cache_size (currently raid5 only) 716 number of entries in the stripe cache. This is writable, but 717 there are upper and lower limits (32768, 17). Default is 256. 718 719 strip_cache_active (currently raid5 only) 720 number of active entries in the stripe cache 721 722 preread_bypass_threshold (currently raid5 only) 723 number of times a stripe requiring preread will be bypassed by 724 a stripe that does not require preread. For fairness defaults 725 to 1. Setting this to 0 disables bypass accounting and 726 requires preread stripes to wait until all full-width stripe- 727 writes are complete. Valid values are 0 to stripe_cache_size. 728 729 journal_mode (currently raid5 only) 730 The cache mode for raid5. raid5 could include an extra disk for 731 caching. The mode can be "write-throuth" and "write-back". The 732 default is "write-through". 733