1Tools that manage md devices can be found at 2 http://www.<country>.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 passed be 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 has 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 it's 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. At array creation it will 237 default to 0, though starting the array as 'clean' will 238 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 300As component devices are added to an md array, they appear in the 'md' 301directory as new directories named 302 dev-XXX 303where XXX is a name that the kernel knows for the device, e.g. hdb1. 304Each directory contains: 305 306 block 307 a symlink to the block device in /sys/block, e.g. 308 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 309 310 super 311 A file containing an image of the superblock read from, or 312 written to, that device. 313 314 state 315 A file recording the current state of the device in the array 316 which can be a comma separated list of 317 faulty - device has been kicked from active use due to 318 a detected fault 319 in_sync - device is a fully in-sync member of the array 320 writemostly - device will only be subject to read 321 requests if there are no other options. 322 This applies only to raid1 arrays. 323 blocked - device has failed, metadata is "external", 324 and the failure hasn't been acknowledged yet. 325 Writes that would write to this device if 326 it were not faulty are blocked. 327 spare - device is working, but not a full member. 328 This includes spares that are in the process 329 of being recovered to 330 This list may grow in future. 331 This can be written to. 332 Writing "faulty" simulates a failure on the device. 333 Writing "remove" removes the device from the array. 334 Writing "writemostly" sets the writemostly flag. 335 Writing "-writemostly" clears the writemostly flag. 336 Writing "blocked" sets the "blocked" flag. 337 Writing "-blocked" clear the "blocked" flag and allows writes 338 to complete. 339 340 This file responds to select/poll. Any change to 'faulty' 341 or 'blocked' causes an event. 342 343 errors 344 An approximate count of read errors that have been detected on 345 this device but have not caused the device to be evicted from 346 the array (either because they were corrected or because they 347 happened while the array was read-only). When using version-1 348 metadata, this value persists across restarts of the array. 349 350 This value can be written while assembling an array thus 351 providing an ongoing count for arrays with metadata managed by 352 userspace. 353 354 slot 355 This gives the role that the device has in the array. It will 356 either be 'none' if the device is not active in the array 357 (i.e. is a spare or has failed) or an integer less than the 358 'raid_disks' number for the array indicating which position 359 it currently fills. This can only be set while assembling an 360 array. A device for which this is set is assumed to be working. 361 362 offset 363 This gives the location in the device (in sectors from the 364 start) where data from the array will be stored. Any part of 365 the device before this offset us not touched, unless it is 366 used for storing metadata (Formats 1.1 and 1.2). 367 368 size 369 The amount of the device, after the offset, that can be used 370 for storage of data. This will normally be the same as the 371 component_size. This can be written while assembling an 372 array. If a value less than the current component_size is 373 written, it will be rejected. 374 375 376An active md device will also contain and entry for each active device 377in the array. These are named 378 379 rdNN 380 381where 'NN' is the position in the array, starting from 0. 382So for a 3 drive array there will be rd0, rd1, rd2. 383These are symbolic links to the appropriate 'dev-XXX' entry. 384Thus, for example, 385 cat /sys/block/md*/md/rd*/state 386will show 'in_sync' on every line. 387 388 389 390Active md devices for levels that support data redundancy (1,4,5,6) 391also have 392 393 sync_action 394 a text file that can be used to monitor and control the rebuild 395 process. It contains one word which can be one of: 396 resync - redundancy is being recalculated after unclean 397 shutdown or creation 398 recover - a hot spare is being built to replace a 399 failed/missing device 400 idle - nothing is happening 401 check - A full check of redundancy was requested and is 402 happening. This reads all block and checks 403 them. A repair may also happen for some raid 404 levels. 405 repair - A full check and repair is happening. This is 406 similar to 'resync', but was requested by the 407 user, and the write-intent bitmap is NOT used to 408 optimise the process. 409 410 This file is writable, and each of the strings that could be 411 read are meaningful for writing. 412 413 'idle' will stop an active resync/recovery etc. There is no 414 guarantee that another resync/recovery may not be automatically 415 started again, though some event will be needed to trigger 416 this. 417 'resync' or 'recovery' can be used to restart the 418 corresponding operation if it was stopped with 'idle'. 419 'check' and 'repair' will start the appropriate process 420 providing the current state is 'idle'. 421 422 This file responds to select/poll. Any important change in the value 423 triggers a poll event. Sometimes the value will briefly be 424 "recover" if a recovery seems to be needed, but cannot be 425 achieved. In that case, the transition to "recover" isn't 426 notified, but the transition away is. 427 428 degraded 429 This contains a count of the number of devices by which the 430 arrays is degraded. So an optimal array with show '0'. A 431 single failed/missing drive will show '1', etc. 432 This file responds to select/poll, any increase or decrease 433 in the count of missing devices will trigger an event. 434 435 mismatch_count 436 When performing 'check' and 'repair', and possibly when 437 performing 'resync', md will count the number of errors that are 438 found. The count in 'mismatch_cnt' is the number of sectors 439 that were re-written, or (for 'check') would have been 440 re-written. As most raid levels work in units of pages rather 441 than sectors, this my be larger than the number of actual errors 442 by a factor of the number of sectors in a page. 443 444 bitmap_set_bits 445 If the array has a write-intent bitmap, then writing to this 446 attribute can set bits in the bitmap, indicating that a resync 447 would need to check the corresponding blocks. Either individual 448 numbers or start-end pairs can be written. Multiple numbers 449 can be separated by a space. 450 Note that the numbers are 'bit' numbers, not 'block' numbers. 451 They should be scaled by the bitmap_chunksize. 452 453 sync_speed_min 454 sync_speed_max 455 This are similar to /proc/sys/dev/raid/speed_limit_{min,max} 456 however they only apply to the particular array. 457 If no value has been written to these, of if the word 'system' 458 is written, then the system-wide value is used. If a value, 459 in kibibytes-per-second is written, then it is used. 460 When the files are read, they show the currently active value 461 followed by "(local)" or "(system)" depending on whether it is 462 a locally set or system-wide value. 463 464 sync_completed 465 This shows the number of sectors that have been completed of 466 whatever the current sync_action is, followed by the number of 467 sectors in total that could need to be processed. The two 468 numbers are separated by a '/' thus effectively showing one 469 value, a fraction of the process that is complete. 470 A 'select' on this attribute will return when resync completes, 471 when it reaches the current sync_max (below) and possibly at 472 other times. 473 474 sync_max 475 This is a number of sectors at which point a resync/recovery 476 process will pause. When a resync is active, the value can 477 only ever be increased, never decreased. The value of 'max' 478 effectively disables the limit. 479 480 481 sync_speed 482 This shows the current actual speed, in K/sec, of the current 483 sync_action. It is averaged over the last 30 seconds. 484 485 suspend_lo 486 suspend_hi 487 The two values, given as numbers of sectors, indicate a range 488 within the array where IO will be blocked. This is currently 489 only supported for raid4/5/6. 490 491 492Each active md device may also have attributes specific to the 493personality module that manages it. 494These are specific to the implementation of the module and could 495change substantially if the implementation changes. 496 497These currently include 498 499 stripe_cache_size (currently raid5 only) 500 number of entries in the stripe cache. This is writable, but 501 there are upper and lower limits (32768, 16). Default is 128. 502 strip_cache_active (currently raid5 only) 503 number of active entries in the stripe cache 504 preread_bypass_threshold (currently raid5 only) 505 number of times a stripe requiring preread will be bypassed by 506 a stripe that does not require preread. For fairness defaults 507 to 1. Setting this to 0 disables bypass accounting and 508 requires preread stripes to wait until all full-width stripe- 509 writes are complete. Valid values are 0 to stripe_cache_size. 510