1RAID arrays 2=========== 3 4Boot time assembly of RAID arrays 5--------------------------------- 6 7Tools that manage md devices can be found at 8 https://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 interpreted 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 logical_block_size 242 Configure the array's logical block size in bytes. This attribute 243 is only supported for 1.x meta. Write the value before starting 244 array. The final array LBS uses the maximum between this 245 configuration and LBS of all combined devices. Note that 246 LBS cannot exceed PAGE_SIZE before RAID supports folio. 247 WARNING: Arrays created on new kernel cannot be assembled at old 248 kernel due to padding check, Set module parameter 'check_new_feature' 249 to false to bypass, but data loss may occur. 250 251 reshape_position 252 This is either ``none`` or a sector number within the devices of 253 the array where ``reshape`` is up to. If this is set, the three 254 attributes mentioned above (raid_disks, chunk_size, layout) can 255 potentially have 2 values, an old and a new value. If these 256 values differ, reading the attribute returns:: 257 258 new (old) 259 260 and writing will effect the ``new`` value, leaving the ``old`` 261 unchanged. 262 263 component_size 264 For arrays with data redundancy (i.e. not raid0, linear, faulty, 265 multipath), all components must be the same size - or at least 266 there must a size that they all provide space for. This is a key 267 part or the geometry of the array. It is measured in sectors 268 and can be read from here. Writing to this value may resize 269 the array if the personality supports it (raid1, raid5, raid6), 270 and if the component drives are large enough. 271 272 metadata_version 273 This indicates the format that is being used to record metadata 274 about the array. It can be 0.90 (traditional format), 1.0, 1.1, 275 1.2 (newer format in varying locations) or ``none`` indicating that 276 the kernel isn't managing metadata at all. 277 Alternately it can be ``external:`` followed by a string which 278 is set by user-space. This indicates that metadata is managed 279 by a user-space program. Any device failure or other event that 280 requires a metadata update will cause array activity to be 281 suspended until the event is acknowledged. 282 283 resync_start 284 The point at which resync should start. If no resync is needed, 285 this will be a very large number (or ``none`` since 2.6.30-rc1). At 286 array creation it will default to 0, though starting the array as 287 ``clean`` will set it much larger. 288 289 new_dev 290 This file can be written but not read. The value written should 291 be a block device number as major:minor. e.g. 8:0 292 This will cause that device to be attached to the array, if it is 293 available. It will then appear at md/dev-XXX (depending on the 294 name of the device) and further configuration is then possible. 295 296 safe_mode_delay 297 When an md array has seen no write requests for a certain period 298 of time, it will be marked as ``clean``. When another write 299 request arrives, the array is marked as ``dirty`` before the write 300 commences. This is known as ``safe_mode``. 301 The ``certain period`` is controlled by this file which stores the 302 period as a number of seconds. The default is 200msec (0.200). 303 Writing a value of 0 disables safemode. 304 305 array_state 306 This file contains a single word which describes the current 307 state of the array. In many cases, the state can be set by 308 writing the word for the desired state, however some states 309 cannot be explicitly set, and some transitions are not allowed. 310 311 Select/poll works on this file. All changes except between 312 Active_idle and active (which can be frequent and are not 313 very interesting) are notified. active->active_idle is 314 reported if the metadata is externally managed. 315 316 clear 317 No devices, no size, no level 318 319 Writing is equivalent to STOP_ARRAY ioctl 320 321 inactive 322 May have some settings, but array is not active 323 all IO results in error 324 325 When written, doesn't tear down array, but just stops it 326 327 suspended (not supported yet) 328 All IO requests will block. The array can be reconfigured. 329 330 Writing this, if accepted, will block until array is quiescent 331 332 readonly 333 no resync can happen. no superblocks get written. 334 335 Write requests fail 336 337 read-auto 338 like readonly, but behaves like ``clean`` on a write request. 339 340 clean 341 no pending writes, but otherwise active. 342 343 When written to inactive array, starts without resync 344 345 If a write request arrives then 346 if metadata is known, mark ``dirty`` and switch to ``active``. 347 if not known, block and switch to write-pending 348 349 If written to an active array that has pending writes, then fails. 350 active 351 fully active: IO and resync can be happening. 352 When written to inactive array, starts with resync 353 354 write-pending 355 clean, but writes are blocked waiting for ``active`` to be written. 356 357 active-idle 358 like active, but no writes have been seen for a while (safe_mode_delay). 359 360 consistency_policy 361 This indicates how the array maintains consistency in case of unexpected 362 shutdown. It can be: 363 364 none 365 Array has no redundancy information, e.g. raid0, linear. 366 367 resync 368 Full resync is performed and all redundancy is regenerated when the 369 array is started after unclean shutdown. 370 371 bitmap 372 Resync assisted by a write-intent bitmap. 373 374 journal 375 For raid4/5/6, journal device is used to log transactions and replay 376 after unclean shutdown. 377 378 ppl 379 For raid5 only, Partial Parity Log is used to close the write hole and 380 eliminate resync. 381 382 The accepted values when writing to this file are ``ppl`` and ``resync``, 383 used to enable and disable PPL. 384 385 uuid 386 This indicates the UUID of the array in the following format: 387 xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx 388 389 bitmap_type 390 [RW] When read, this file will display the current and available 391 bitmap for this array. The currently active bitmap will be enclosed 392 in [] brackets. Writing an bitmap name or ID to this file will switch 393 control of this array to that new bitmap. Note that writing a new 394 bitmap for created array is forbidden. 395 396 none 397 No bitmap 398 bitmap 399 The default internal bitmap 400 llbitmap 401 The lockless internal bitmap 402 403If bitmap_type is not none, then additional bitmap attributes bitmap/xxx or 404llbitmap/xxx will be created after md device KOBJ_CHANGE event. 405 406If bitmap_type is bitmap, then the md device will also contain: 407 408 bitmap/location 409 This indicates where the write-intent bitmap for the array is 410 stored. 411 412 It can be one of ``none``, ``file`` or ``[+-]N``. 413 ``file`` may later be extended to ``file:/file/name`` 414 ``[+-]N`` means that many sectors from the start of the metadata. 415 416 This is replicated on all devices. For arrays with externally 417 managed metadata, the offset is from the beginning of the 418 device. 419 420 bitmap/chunksize 421 The size, in bytes, of the chunk which will be represented by a 422 single bit. For RAID456, it is a portion of an individual 423 device. For RAID10, it is a portion of the array. For RAID1, it 424 is both (they come to the same thing). 425 426 bitmap/time_base 427 The time, in seconds, between looking for bits in the bitmap to 428 be cleared. In the current implementation, a bit will be cleared 429 between 2 and 3 times ``time_base`` after all the covered blocks 430 are known to be in-sync. 431 432 bitmap/backlog 433 When write-mostly devices are active in a RAID1, write requests 434 to those devices proceed in the background - the filesystem (or 435 other user of the device) does not have to wait for them. 436 ``backlog`` sets a limit on the number of concurrent background 437 writes. If there are more than this, new writes will by 438 synchronous. 439 440 bitmap/metadata 441 This can be either ``internal`` or ``external``. 442 443 ``internal`` 444 is the default and means the metadata for the bitmap 445 is stored in the first 256 bytes of the allocated space and is 446 managed by the md module. 447 448 ``external`` 449 means that bitmap metadata is managed externally to 450 the kernel (i.e. by some userspace program) 451 452 bitmap/can_clear 453 This is either ``true`` or ``false``. If ``true``, then bits in the 454 bitmap will be cleared when the corresponding blocks are thought 455 to be in-sync. If ``false``, bits will never be cleared. 456 This is automatically set to ``false`` if a write happens on a 457 degraded array, or if the array becomes degraded during a write. 458 When metadata is managed externally, it should be set to true 459 once the array becomes non-degraded, and this fact has been 460 recorded in the metadata. 461 462If bitmap_type is llbitmap, then the md device will also contain: 463 464 llbitmap/bits 465 This is read-only, show status of bitmap bits, the number of each 466 value. 467 468 llbitmap/metadata 469 This is read-only, show bitmap metadata, include chunksize, chunkshift, 470 chunks, offset and daemon_sleep. 471 472 llbitmap/daemon_sleep 473 This is read-write, time in seconds that daemon function will be 474 triggered to clear dirty bits. 475 476 llbitmap/barrier_idle 477 This is read-write, time in seconds that page barrier will be idled, 478 means dirty bits in the page will be cleared. 479 480As component devices are added to an md array, they appear in the ``md`` 481directory as new directories named:: 482 483 dev-XXX 484 485where ``XXX`` is a name that the kernel knows for the device, e.g. hdb1. 486Each directory contains: 487 488 block 489 a symlink to the block device in /sys/block, e.g.:: 490 491 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 492 493 super 494 A file containing an image of the superblock read from, or 495 written to, that device. 496 497 state 498 A file recording the current state of the device in the array 499 which can be a comma separated list of: 500 501 faulty 502 device has been kicked from active use due to 503 a detected fault, or it has unacknowledged bad 504 blocks 505 506 in_sync 507 device is a fully in-sync member of the array 508 509 writemostly 510 device will only be subject to read 511 requests if there are no other options. 512 513 This applies only to raid1 arrays. 514 515 blocked 516 device has failed, and the failure hasn't been 517 acknowledged yet by the metadata handler. 518 519 Writes that would write to this device if 520 it were not faulty are blocked. 521 522 spare 523 device is working, but not a full member. 524 525 This includes spares that are in the process 526 of being recovered to 527 528 write_error 529 device has ever seen a write error. 530 531 want_replacement 532 device is (mostly) working but probably 533 should be replaced, either due to errors or 534 due to user request. 535 536 replacement 537 device is a replacement for another active 538 device with same raid_disk. 539 540 541 This list may grow in future. 542 543 This can be written to. 544 545 Writing ``faulty`` simulates a failure on the device. 546 547 Writing ``remove`` removes the device from the array. 548 549 Writing ``writemostly`` sets the writemostly flag. 550 551 Writing ``-writemostly`` clears the writemostly flag. 552 553 Writing ``blocked`` sets the ``blocked`` flag. 554 555 Writing ``-blocked`` clears the ``blocked`` flags and allows writes 556 to complete and possibly simulates an error. 557 558 Writing ``in_sync`` sets the in_sync flag. 559 560 Writing ``write_error`` sets writeerrorseen flag. 561 562 Writing ``-write_error`` clears writeerrorseen flag. 563 564 Writing ``want_replacement`` is allowed at any time except to a 565 replacement device or a spare. It sets the flag. 566 567 Writing ``-want_replacement`` is allowed at any time. It clears 568 the flag. 569 570 Writing ``replacement`` or ``-replacement`` is only allowed before 571 starting the array. It sets or clears the flag. 572 573 574 This file responds to select/poll. Any change to ``faulty`` 575 or ``blocked`` causes an event. 576 577 errors 578 An approximate count of read errors that have been detected on 579 this device but have not caused the device to be evicted from 580 the array (either because they were corrected or because they 581 happened while the array was read-only). When using version-1 582 metadata, this value persists across restarts of the array. 583 584 This value can be written while assembling an array thus 585 providing an ongoing count for arrays with metadata managed by 586 userspace. 587 588 slot 589 This gives the role that the device has in the array. It will 590 either be ``none`` if the device is not active in the array 591 (i.e. is a spare or has failed) or an integer less than the 592 ``raid_disks`` number for the array indicating which position 593 it currently fills. This can only be set while assembling an 594 array. A device for which this is set is assumed to be working. 595 596 offset 597 This gives the location in the device (in sectors from the 598 start) where data from the array will be stored. Any part of 599 the device before this offset is not touched, unless it is 600 used for storing metadata (Formats 1.1 and 1.2). 601 602 size 603 The amount of the device, after the offset, that can be used 604 for storage of data. This will normally be the same as the 605 component_size. This can be written while assembling an 606 array. If a value less than the current component_size is 607 written, it will be rejected. 608 609 recovery_start 610 When the device is not ``in_sync``, this records the number of 611 sectors from the start of the device which are known to be 612 correct. This is normally zero, but during a recovery 613 operation it will steadily increase, and if the recovery is 614 interrupted, restoring this value can cause recovery to 615 avoid repeating the earlier blocks. With v1.x metadata, this 616 value is saved and restored automatically. 617 618 This can be set whenever the device is not an active member of 619 the array, either before the array is activated, or before 620 the ``slot`` is set. 621 622 Setting this to ``none`` is equivalent to setting ``in_sync``. 623 Setting to any other value also clears the ``in_sync`` flag. 624 625 bad_blocks 626 This gives the list of all known bad blocks in the form of 627 start address and length (in sectors respectively). If output 628 is too big to fit in a page, it will be truncated. Writing 629 ``sector length`` to this file adds new acknowledged (i.e. 630 recorded to disk safely) bad blocks. 631 632 unacknowledged_bad_blocks 633 This gives the list of known-but-not-yet-saved-to-disk bad 634 blocks in the same form of ``bad_blocks``. If output is too big 635 to fit in a page, it will be truncated. Writing to this file 636 adds bad blocks without acknowledging them. This is largely 637 for testing. 638 639 ppl_sector, ppl_size 640 Location and size (in sectors) of the space used for Partial Parity Log 641 on this device. 642 643 644An active md device will also contain an entry for each active device 645in the array. These are named:: 646 647 rdNN 648 649where ``NN`` is the position in the array, starting from 0. 650So for a 3 drive array there will be rd0, rd1, rd2. 651These are symbolic links to the appropriate ``dev-XXX`` entry. 652Thus, for example:: 653 654 cat /sys/block/md*/md/rd*/state 655 656will show ``in_sync`` on every line. 657 658 659 660Active md devices for levels that support data redundancy (1,4,5,6,10) 661also have 662 663 sync_action 664 a text file that can be used to monitor and control the rebuild 665 process. It contains one word which can be one of: 666 667 resync 668 redundancy is being recalculated after unclean 669 shutdown or creation 670 671 recover 672 a hot spare is being built to replace a 673 failed/missing device 674 675 idle 676 nothing is happening 677 check 678 A full check of redundancy was requested and is 679 happening. This reads all blocks and checks 680 them. A repair may also happen for some raid 681 levels. 682 683 repair 684 A full check and repair is happening. This is 685 similar to ``resync``, but was requested by the 686 user, and the write-intent bitmap is NOT used to 687 optimise the process. 688 689 This file is writable, and each of the strings that could be 690 read are meaningful for writing. 691 692 ``idle`` will stop an active resync/recovery etc. There is no 693 guarantee that another resync/recovery may not be automatically 694 started again, though some event will be needed to trigger 695 this. 696 697 ``resync`` or ``recovery`` can be used to restart the 698 corresponding operation if it was stopped with ``idle``. 699 700 ``check`` and ``repair`` will start the appropriate process 701 providing the current state is ``idle``. 702 703 This file responds to select/poll. Any important change in the value 704 triggers a poll event. Sometimes the value will briefly be 705 ``recover`` if a recovery seems to be needed, but cannot be 706 achieved. In that case, the transition to ``recover`` isn't 707 notified, but the transition away is. 708 709 degraded 710 This contains a count of the number of devices by which the 711 arrays is degraded. So an optimal array will show ``0``. A 712 single failed/missing drive will show ``1``, etc. 713 714 This file responds to select/poll, any increase or decrease 715 in the count of missing devices will trigger an event. 716 717 mismatch_count 718 When performing ``check`` and ``repair``, and possibly when 719 performing ``resync``, md will count the number of errors that are 720 found. The count in ``mismatch_cnt`` is the number of sectors 721 that were re-written, or (for ``check``) would have been 722 re-written. As most raid levels work in units of pages rather 723 than sectors, this may be larger than the number of actual errors 724 by a factor of the number of sectors in a page. 725 726 bitmap_set_bits 727 If the array has a write-intent bitmap, then writing to this 728 attribute can set bits in the bitmap, indicating that a resync 729 would need to check the corresponding blocks. Either individual 730 numbers or start-end pairs can be written. Multiple numbers 731 can be separated by a space. 732 733 Note that the numbers are ``bit`` numbers, not ``block`` numbers. 734 They should be scaled by the bitmap_chunksize. 735 736 sync_speed_min, sync_speed_max 737 This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}`` 738 however they only apply to the particular array. 739 740 If no value has been written to these, or if the word ``system`` 741 is written, then the system-wide value is used. If a value, 742 in kibibytes-per-second is written, then it is used. 743 744 When the files are read, they show the currently active value 745 followed by ``(local)`` or ``(system)`` depending on whether it is 746 a locally set or system-wide value. 747 748 sync_completed 749 This shows the number of sectors that have been completed of 750 whatever the current sync_action is, followed by the number of 751 sectors in total that could need to be processed. The two 752 numbers are separated by a ``/`` thus effectively showing one 753 value, a fraction of the process that is complete. 754 755 A ``select`` on this attribute will return when resync completes, 756 when it reaches the current sync_max (below) and possibly at 757 other times. 758 759 sync_speed 760 This shows the current actual speed, in K/sec, of the current 761 sync_action. It is averaged over the last 30 seconds. 762 763 suspend_lo, suspend_hi 764 The two values, given as numbers of sectors, indicate a range 765 within the array where IO will be blocked. This is currently 766 only supported for raid4/5/6. 767 768 sync_min, sync_max 769 The two values, given as numbers of sectors, indicate a range 770 within the array where ``check``/``repair`` will operate. Must be 771 a multiple of chunk_size. When it reaches ``sync_max`` it will 772 pause, rather than complete. 773 You can use ``select`` or ``poll`` on ``sync_completed`` to wait for 774 that number to reach sync_max. Then you can either increase 775 ``sync_max``, or can write ``idle`` to ``sync_action``. 776 777 The value of ``max`` for ``sync_max`` effectively disables the limit. 778 When a resync is active, the value can only ever be increased, 779 never decreased. 780 The value of ``0`` is the minimum for ``sync_min``. 781 782 783 784Each active md device may also have attributes specific to the 785personality module that manages it. 786These are specific to the implementation of the module and could 787change substantially if the implementation changes. 788 789These currently include: 790 791 stripe_cache_size (currently raid5 only) 792 number of entries in the stripe cache. This is writable, but 793 there are upper and lower limits (32768, 17). Default is 256. 794 795 strip_cache_active (currently raid5 only) 796 number of active entries in the stripe cache 797 798 preread_bypass_threshold (currently raid5 only) 799 number of times a stripe requiring preread will be bypassed by 800 a stripe that does not require preread. For fairness defaults 801 to 1. Setting this to 0 disables bypass accounting and 802 requires preread stripes to wait until all full-width stripe- 803 writes are complete. Valid values are 0 to stripe_cache_size. 804 805 journal_mode (currently raid5 only) 806 The cache mode for raid5. raid5 could include an extra disk for 807 caching. The mode can be "write-through" or "write-back". The 808 default is "write-through". 809 810 ppl_write_hint 811 NVMe stream ID to be set for each PPL write request. 812