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 (not currently 168 possible) this will contain the larger of the old and new sizes. 169 Some raid level (RAID1) allow this value to be set while the 170 array is active. This will reconfigure the array. Otherwise 171 it can only be set while assembling an array. 172 173 chunk_size 174 This is the size if bytes for 'chunks' and is only relevant to 175 raid levels that involve striping (1,4,5,6,10). The address space 176 of the array is conceptually divided into chunks and consecutive 177 chunks are striped onto neighbouring devices. 178 The size should be at least PAGE_SIZE (4k) and should be a power 179 of 2. This can only be set while assembling an array 180 181 layout 182 The "layout" for the array for the particular level. This is 183 simply a number that is interpretted differently by different 184 levels. It can be written while assembling an array. 185 186 reshape_position 187 This is either "none" or a sector number within the devices of 188 the array where "reshape" is up to. If this is set, the three 189 attributes mentioned above (raid_disks, chunk_size, layout) can 190 potentially have 2 values, an old and a new value. If these 191 values differ, reading the attribute returns 192 new (old) 193 and writing will effect the 'new' value, leaving the 'old' 194 unchanged. 195 196 component_size 197 For arrays with data redundancy (i.e. not raid0, linear, faulty, 198 multipath), all components must be the same size - or at least 199 there must a size that they all provide space for. This is a key 200 part or the geometry of the array. It is measured in sectors 201 and can be read from here. Writing to this value may resize 202 the array if the personality supports it (raid1, raid5, raid6), 203 and if the component drives are large enough. 204 205 metadata_version 206 This indicates the format that is being used to record metadata 207 about the array. It can be 0.90 (traditional format), 1.0, 1.1, 208 1.2 (newer format in varying locations) or "none" indicating that 209 the kernel isn't managing metadata at all. 210 211 resync_start 212 The point at which resync should start. If no resync is needed, 213 this will be a very large number. At array creation it will 214 default to 0, though starting the array as 'clean' will 215 set it much larger. 216 217 new_dev 218 This file can be written but not read. The value written should 219 be a block device number as major:minor. e.g. 8:0 220 This will cause that device to be attached to the array, if it is 221 available. It will then appear at md/dev-XXX (depending on the 222 name of the device) and further configuration is then possible. 223 224 safe_mode_delay 225 When an md array has seen no write requests for a certain period 226 of time, it will be marked as 'clean'. When another write 227 request arrives, the array is marked as 'dirty' before the write 228 commences. This is known as 'safe_mode'. 229 The 'certain period' is controlled by this file which stores the 230 period as a number of seconds. The default is 200msec (0.200). 231 Writing a value of 0 disables safemode. 232 233 array_state 234 This file contains a single word which describes the current 235 state of the array. In many cases, the state can be set by 236 writing the word for the desired state, however some states 237 cannot be explicitly set, and some transitions are not allowed. 238 239 Select/poll works on this file. All changes except between 240 active_idle and active (which can be frequent and are not 241 very interesting) are notified. active->active_idle is 242 reported if the metadata is externally managed. 243 244 clear 245 No devices, no size, no level 246 Writing is equivalent to STOP_ARRAY ioctl 247 inactive 248 May have some settings, but array is not active 249 all IO results in error 250 When written, doesn't tear down array, but just stops it 251 suspended (not supported yet) 252 All IO requests will block. The array can be reconfigured. 253 Writing this, if accepted, will block until array is quiessent 254 readonly 255 no resync can happen. no superblocks get written. 256 write requests fail 257 read-auto 258 like readonly, but behaves like 'clean' on a write request. 259 260 clean - no pending writes, but otherwise active. 261 When written to inactive array, starts without resync 262 If a write request arrives then 263 if metadata is known, mark 'dirty' and switch to 'active'. 264 if not known, block and switch to write-pending 265 If written to an active array that has pending writes, then fails. 266 active 267 fully active: IO and resync can be happening. 268 When written to inactive array, starts with resync 269 270 write-pending 271 clean, but writes are blocked waiting for 'active' to be written. 272 273 active-idle 274 like active, but no writes have been seen for a while (safe_mode_delay). 275 276 277As component devices are added to an md array, they appear in the 'md' 278directory as new directories named 279 dev-XXX 280where XXX is a name that the kernel knows for the device, e.g. hdb1. 281Each directory contains: 282 283 block 284 a symlink to the block device in /sys/block, e.g. 285 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 286 287 super 288 A file containing an image of the superblock read from, or 289 written to, that device. 290 291 state 292 A file recording the current state of the device in the array 293 which can be a comma separated list of 294 faulty - device has been kicked from active use due to 295 a detected fault 296 in_sync - device is a fully in-sync member of the array 297 writemostly - device will only be subject to read 298 requests if there are no other options. 299 This applies only to raid1 arrays. 300 blocked - device has failed, metadata is "external", 301 and the failure hasn't been acknowledged yet. 302 Writes that would write to this device if 303 it were not faulty are blocked. 304 spare - device is working, but not a full member. 305 This includes spares that are in the process 306 of being recovered to 307 This list may grow in future. 308 This can be written to. 309 Writing "faulty" simulates a failure on the device. 310 Writing "remove" removes the device from the array. 311 Writing "writemostly" sets the writemostly flag. 312 Writing "-writemostly" clears the writemostly flag. 313 Writing "blocked" sets the "blocked" flag. 314 Writing "-blocked" clear the "blocked" flag and allows writes 315 to complete. 316 317 This file responds to select/poll. Any change to 'faulty' 318 or 'blocked' causes an event. 319 320 errors 321 An approximate count of read errors that have been detected on 322 this device but have not caused the device to be evicted from 323 the array (either because they were corrected or because they 324 happened while the array was read-only). When using version-1 325 metadata, this value persists across restarts of the array. 326 327 This value can be written while assembling an array thus 328 providing an ongoing count for arrays with metadata managed by 329 userspace. 330 331 slot 332 This gives the role that the device has in the array. It will 333 either be 'none' if the device is not active in the array 334 (i.e. is a spare or has failed) or an integer less than the 335 'raid_disks' number for the array indicating which position 336 it currently fills. This can only be set while assembling an 337 array. A device for which this is set is assumed to be working. 338 339 offset 340 This gives the location in the device (in sectors from the 341 start) where data from the array will be stored. Any part of 342 the device before this offset us not touched, unless it is 343 used for storing metadata (Formats 1.1 and 1.2). 344 345 size 346 The amount of the device, after the offset, that can be used 347 for storage of data. This will normally be the same as the 348 component_size. This can be written while assembling an 349 array. If a value less than the current component_size is 350 written, it will be rejected. 351 352 353An active md device will also contain and entry for each active device 354in the array. These are named 355 356 rdNN 357 358where 'NN' is the position in the array, starting from 0. 359So for a 3 drive array there will be rd0, rd1, rd2. 360These are symbolic links to the appropriate 'dev-XXX' entry. 361Thus, for example, 362 cat /sys/block/md*/md/rd*/state 363will show 'in_sync' on every line. 364 365 366 367Active md devices for levels that support data redundancy (1,4,5,6) 368also have 369 370 sync_action 371 a text file that can be used to monitor and control the rebuild 372 process. It contains one word which can be one of: 373 resync - redundancy is being recalculated after unclean 374 shutdown or creation 375 recover - a hot spare is being built to replace a 376 failed/missing device 377 idle - nothing is happening 378 check - A full check of redundancy was requested and is 379 happening. This reads all block and checks 380 them. A repair may also happen for some raid 381 levels. 382 repair - A full check and repair is happening. This is 383 similar to 'resync', but was requested by the 384 user, and the write-intent bitmap is NOT used to 385 optimise the process. 386 387 This file is writable, and each of the strings that could be 388 read are meaningful for writing. 389 390 'idle' will stop an active resync/recovery etc. There is no 391 guarantee that another resync/recovery may not be automatically 392 started again, though some event will be needed to trigger 393 this. 394 'resync' or 'recovery' can be used to restart the 395 corresponding operation if it was stopped with 'idle'. 396 'check' and 'repair' will start the appropriate process 397 providing the current state is 'idle'. 398 399 This file responds to select/poll. Any important change in the value 400 triggers a poll event. Sometimes the value will briefly be 401 "recover" if a recovery seems to be needed, but cannot be 402 achieved. In that case, the transition to "recover" isn't 403 notified, but the transition away is. 404 405 degraded 406 This contains a count of the number of devices by which the 407 arrays is degraded. So an optimal array with show '0'. A 408 single failed/missing drive will show '1', etc. 409 This file responds to select/poll, any increase or decrease 410 in the count of missing devices will trigger an event. 411 412 mismatch_count 413 When performing 'check' and 'repair', and possibly when 414 performing 'resync', md will count the number of errors that are 415 found. The count in 'mismatch_cnt' is the number of sectors 416 that were re-written, or (for 'check') would have been 417 re-written. As most raid levels work in units of pages rather 418 than sectors, this my be larger than the number of actual errors 419 by a factor of the number of sectors in a page. 420 421 bitmap_set_bits 422 If the array has a write-intent bitmap, then writing to this 423 attribute can set bits in the bitmap, indicating that a resync 424 would need to check the corresponding blocks. Either individual 425 numbers or start-end pairs can be written. Multiple numbers 426 can be separated by a space. 427 Note that the numbers are 'bit' numbers, not 'block' numbers. 428 They should be scaled by the bitmap_chunksize. 429 430 sync_speed_min 431 sync_speed_max 432 This are similar to /proc/sys/dev/raid/speed_limit_{min,max} 433 however they only apply to the particular array. 434 If no value has been written to these, of if the word 'system' 435 is written, then the system-wide value is used. If a value, 436 in kibibytes-per-second is written, then it is used. 437 When the files are read, they show the currently active value 438 followed by "(local)" or "(system)" depending on whether it is 439 a locally set or system-wide value. 440 441 sync_completed 442 This shows the number of sectors that have been completed of 443 whatever the current sync_action is, followed by the number of 444 sectors in total that could need to be processed. The two 445 numbers are separated by a '/' thus effectively showing one 446 value, a fraction of the process that is complete. 447 A 'select' on this attribute will return when resync completes, 448 when it reaches the current sync_max (below) and possibly at 449 other times. 450 451 sync_max 452 This is a number of sectors at which point a resync/recovery 453 process will pause. When a resync is active, the value can 454 only ever be increased, never decreased. The value of 'max' 455 effectively disables the limit. 456 457 458 sync_speed 459 This shows the current actual speed, in K/sec, of the current 460 sync_action. It is averaged over the last 30 seconds. 461 462 suspend_lo 463 suspend_hi 464 The two values, given as numbers of sectors, indicate a range 465 within the array where IO will be blocked. This is currently 466 only supported for raid4/5/6. 467 468 469Each active md device may also have attributes specific to the 470personality module that manages it. 471These are specific to the implementation of the module and could 472change substantially if the implementation changes. 473 474These currently include 475 476 stripe_cache_size (currently raid5 only) 477 number of entries in the stripe cache. This is writable, but 478 there are upper and lower limits (32768, 16). Default is 128. 479 strip_cache_active (currently raid5 only) 480 number of active entries in the stripe cache 481 preread_bypass_threshold (currently raid5 only) 482 number of times a stripe requiring preread will be bypassed by 483 a stripe that does not require preread. For fairness defaults 484 to 1. Setting this to 0 disables bypass accounting and 485 requires preread stripes to wait until all full-width stripe- 486 writes are complete. Valid values are 0 to stripe_cache_size. 487