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