diff options
Diffstat (limited to 'Documentation/md.txt')
| -rw-r--r-- | Documentation/md.txt | 583 |
1 files changed, 583 insertions, 0 deletions
diff --git a/Documentation/md.txt b/Documentation/md.txt new file mode 100644 index 00000000..f0eee83f --- /dev/null +++ b/Documentation/md.txt @@ -0,0 +1,583 @@ +Tools that manage md devices can be found at + http://www.kernel.org/pub/linux/utils/raid/ + + +Boot time assembly of RAID arrays +--------------------------------- + +You can boot with your md device with the following kernel command +lines: + +for old raid arrays without persistent superblocks: + md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn + +for raid arrays with persistent superblocks + md=<md device no.>,dev0,dev1,...,devn +or, to assemble a partitionable array: + md=d<md device no.>,dev0,dev1,...,devn + +md device no. = the number of the md device ... + 0 means md0, + 1 md1, + 2 md2, + 3 md3, + 4 md4 + +raid level = -1 linear mode + 0 striped mode + other modes are only supported with persistent super blocks + +chunk size factor = (raid-0 and raid-1 only) + Set the chunk size as 4k << n. + +fault level = totally ignored + +dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1 + +A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this: + +e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro + + +Boot time autodetection of RAID arrays +-------------------------------------- + +When md is compiled into the kernel (not as module), partitions of +type 0xfd are scanned and automatically assembled into RAID arrays. +This autodetection may be suppressed with the kernel parameter +"raid=noautodetect". As of kernel 2.6.9, only drives with a type 0 +superblock can be autodetected and run at boot time. + +The kernel parameter "raid=partitionable" (or "raid=part") means +that all auto-detected arrays are assembled as partitionable. + +Boot time assembly of degraded/dirty arrays +------------------------------------------- + +If a raid5 or raid6 array is both dirty and degraded, it could have +undetectable data corruption. This is because the fact that it is +'dirty' means that the parity cannot be trusted, and the fact that it +is degraded means that some datablocks are missing and cannot reliably +be reconstructed (due to no parity). + +For this reason, md will normally refuse to start such an array. This +requires the sysadmin to take action to explicitly start the array +despite possible corruption. This is normally done with + mdadm --assemble --force .... + +This option is not really available if the array has the root +filesystem on it. In order to support this booting from such an +array, md supports a module parameter "start_dirty_degraded" which, +when set to 1, bypassed the checks and will allows dirty degraded +arrays to be started. + +So, to boot with a root filesystem of a dirty degraded raid[56], use + + md-mod.start_dirty_degraded=1 + + +Superblock formats +------------------ + +The md driver can support a variety of different superblock formats. +Currently, it supports superblock formats "0.90.0" and the "md-1" format +introduced in the 2.5 development series. + +The kernel will autodetect which format superblock is being used. + +Superblock format '0' is treated differently to others for legacy +reasons - it is the original superblock format. + + +General Rules - apply for all superblock formats +------------------------------------------------ + +An array is 'created' by writing appropriate superblocks to all +devices. + +It is 'assembled' by associating each of these devices with an +particular md virtual device. Once it is completely assembled, it can +be accessed. + +An array should be created by a user-space tool. This will write +superblocks to all devices. It will usually mark the array as +'unclean', or with some devices missing so that the kernel md driver +can create appropriate redundancy (copying in raid1, parity +calculation in raid4/5). + +When an array is assembled, it is first initialized with the +SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor +version number. The major version number selects which superblock +format is to be used. The minor number might be used to tune handling +of the format, such as suggesting where on each device to look for the +superblock. + +Then each device is added using the ADD_NEW_DISK ioctl. This +provides, in particular, a major and minor number identifying the +device to add. + +The array is started with the RUN_ARRAY ioctl. + +Once started, new devices can be added. They should have an +appropriate superblock written to them, and then passed be in with +ADD_NEW_DISK. + +Devices that have failed or are not yet active can be detached from an +array using HOT_REMOVE_DISK. + + +Specific Rules that apply to format-0 super block arrays, and + arrays with no superblock (non-persistent). +------------------------------------------------------------- + +An array can be 'created' by describing the array (level, chunksize +etc) in a SET_ARRAY_INFO ioctl. This must has major_version==0 and +raid_disks != 0. + +Then uninitialized devices can be added with ADD_NEW_DISK. The +structure passed to ADD_NEW_DISK must specify the state of the device +and its role in the array. + +Once started with RUN_ARRAY, uninitialized spares can be added with +HOT_ADD_DISK. + + + +MD devices in sysfs +------------------- +md devices appear in sysfs (/sys) as regular block devices, +e.g. + /sys/block/md0 + +Each 'md' device will contain a subdirectory called 'md' which +contains further md-specific information about the device. + +All md devices contain: + level + a text file indicating the 'raid level'. e.g. raid0, raid1, + raid5, linear, multipath, faulty. + If no raid level has been set yet (array is still being + assembled), the value will reflect whatever has been written + to it, which may be a name like the above, or may be a number + such as '0', '5', etc. + + raid_disks + a text file with a simple number indicating the number of devices + in a fully functional array. If this is not yet known, the file + will be empty. If an array is being resized this will contain + the new number of devices. + Some raid levels allow this value to be set while the array is + active. This will reconfigure the array. Otherwise it can only + be set while assembling an array. + A change to this attribute will not be permitted if it would + reduce the size of the array. To reduce the number of drives + in an e.g. raid5, the array size must first be reduced by + setting the 'array_size' attribute. + + chunk_size + This is the size in bytes for 'chunks' and is only relevant to + raid levels that involve striping (0,4,5,6,10). The address space + of the array is conceptually divided into chunks and consecutive + chunks are striped onto neighbouring devices. + The size should be at least PAGE_SIZE (4k) and should be a power + of 2. This can only be set while assembling an array + + layout + The "layout" for the array for the particular level. This is + simply a number that is interpretted differently by different + levels. It can be written while assembling an array. + + array_size + This can be used to artificially constrain the available space in + the array to be less than is actually available on the combined + devices. Writing a number (in Kilobytes) which is less than + the available size will set the size. Any reconfiguration of the + array (e.g. adding devices) will not cause the size to change. + Writing the word 'default' will cause the effective size of the + array to be whatever size is actually available based on + 'level', 'chunk_size' and 'component_size'. + + This can be used to reduce the size of the array before reducing + the number of devices in a raid4/5/6, or to support external + metadata formats which mandate such clipping. + + reshape_position + This is either "none" or a sector number within the devices of + the array where "reshape" is up to. If this is set, the three + attributes mentioned above (raid_disks, chunk_size, layout) can + potentially have 2 values, an old and a new value. If these + values differ, reading the attribute returns + new (old) + and writing will effect the 'new' value, leaving the 'old' + unchanged. + + component_size + For arrays with data redundancy (i.e. not raid0, linear, faulty, + multipath), all components must be the same size - or at least + there must a size that they all provide space for. This is a key + part or the geometry of the array. It is measured in sectors + and can be read from here. Writing to this value may resize + the array if the personality supports it (raid1, raid5, raid6), + and if the component drives are large enough. + + metadata_version + This indicates the format that is being used to record metadata + about the array. It can be 0.90 (traditional format), 1.0, 1.1, + 1.2 (newer format in varying locations) or "none" indicating that + the kernel isn't managing metadata at all. + Alternately it can be "external:" followed by a string which + is set by user-space. This indicates that metadata is managed + by a user-space program. Any device failure or other event that + requires a metadata update will cause array activity to be + suspended until the event is acknowledged. + + resync_start + The point at which resync should start. If no resync is needed, + this will be a very large number (or 'none' since 2.6.30-rc1). At + array creation it will default to 0, though starting the array as + 'clean' will set it much larger. + + new_dev + This file can be written but not read. The value written should + be a block device number as major:minor. e.g. 8:0 + This will cause that device to be attached to the array, if it is + available. It will then appear at md/dev-XXX (depending on the + name of the device) and further configuration is then possible. + + safe_mode_delay + When an md array has seen no write requests for a certain period + of time, it will be marked as 'clean'. When another write + request arrives, the array is marked as 'dirty' before the write + commences. This is known as 'safe_mode'. + The 'certain period' is controlled by this file which stores the + period as a number of seconds. The default is 200msec (0.200). + Writing a value of 0 disables safemode. + + array_state + This file contains a single word which describes the current + state of the array. In many cases, the state can be set by + writing the word for the desired state, however some states + cannot be explicitly set, and some transitions are not allowed. + + Select/poll works on this file. All changes except between + active_idle and active (which can be frequent and are not + very interesting) are notified. active->active_idle is + reported if the metadata is externally managed. + + clear + No devices, no size, no level + Writing is equivalent to STOP_ARRAY ioctl + inactive + May have some settings, but array is not active + all IO results in error + When written, doesn't tear down array, but just stops it + suspended (not supported yet) + All IO requests will block. The array can be reconfigured. + Writing this, if accepted, will block until array is quiessent + readonly + no resync can happen. no superblocks get written. + write requests fail + read-auto + like readonly, but behaves like 'clean' on a write request. + + clean - no pending writes, but otherwise active. + When written to inactive array, starts without resync + If a write request arrives then + if metadata is known, mark 'dirty' and switch to 'active'. + if not known, block and switch to write-pending + If written to an active array that has pending writes, then fails. + active + fully active: IO and resync can be happening. + When written to inactive array, starts with resync + + write-pending + clean, but writes are blocked waiting for 'active' to be written. + + active-idle + like active, but no writes have been seen for a while (safe_mode_delay). + + bitmap/location + This indicates where the write-intent bitmap for the array is + stored. + It can be one of "none", "file" or "[+-]N". + "file" may later be extended to "file:/file/name" + "[+-]N" means that many sectors from the start of the metadata. + This is replicated on all devices. For arrays with externally + managed metadata, the offset is from the beginning of the + device. + bitmap/chunksize + The size, in bytes, of the chunk which will be represented by a + single bit. For RAID456, it is a portion of an individual + device. For RAID10, it is a portion of the array. For RAID1, it + is both (they come to the same thing). + bitmap/time_base + The time, in seconds, between looking for bits in the bitmap to + be cleared. In the current implementation, a bit will be cleared + between 2 and 3 times "time_base" after all the covered blocks + are known to be in-sync. + bitmap/backlog + When write-mostly devices are active in a RAID1, write requests + to those devices proceed in the background - the filesystem (or + other user of the device) does not have to wait for them. + 'backlog' sets a limit on the number of concurrent background + writes. If there are more than this, new writes will by + synchronous. + bitmap/metadata + This can be either 'internal' or 'external'. + 'internal' is the default and means the metadata for the bitmap + is stored in the first 256 bytes of the allocated space and is + managed by the md module. + 'external' means that bitmap metadata is managed externally to + the kernel (i.e. by some userspace program) + bitmap/can_clear + This is either 'true' or 'false'. If 'true', then bits in the + bitmap will be cleared when the corresponding blocks are thought + to be in-sync. If 'false', bits will never be cleared. + This is automatically set to 'false' if a write happens on a + degraded array, or if the array becomes degraded during a write. + When metadata is managed externally, it should be set to true + once the array becomes non-degraded, and this fact has been + recorded in the metadata. + + + + +As component devices are added to an md array, they appear in the 'md' +directory as new directories named + dev-XXX +where XXX is a name that the kernel knows for the device, e.g. hdb1. +Each directory contains: + + block + a symlink to the block device in /sys/block, e.g. + /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 + + super + A file containing an image of the superblock read from, or + written to, that device. + + state + A file recording the current state of the device in the array + which can be a comma separated list of + faulty - device has been kicked from active use due to + a detected fault + in_sync - device is a fully in-sync member of the array + writemostly - device will only be subject to read + requests if there are no other options. + This applies only to raid1 arrays. + blocked - device has failed, metadata is "external", + and the failure hasn't been acknowledged yet. + Writes that would write to this device if + it were not faulty are blocked. + spare - device is working, but not a full member. + This includes spares that are in the process + of being recovered to + This list may grow in future. + This can be written to. + Writing "faulty" simulates a failure on the device. + Writing "remove" removes the device from the array. + Writing "writemostly" sets the writemostly flag. + Writing "-writemostly" clears the writemostly flag. + Writing "blocked" sets the "blocked" flag. + Writing "-blocked" clears the "blocked" flag and allows writes + to complete. + Writing "in_sync" sets the in_sync flag. + + This file responds to select/poll. Any change to 'faulty' + or 'blocked' causes an event. + + errors + An approximate count of read errors that have been detected on + this device but have not caused the device to be evicted from + the array (either because they were corrected or because they + happened while the array was read-only). When using version-1 + metadata, this value persists across restarts of the array. + + This value can be written while assembling an array thus + providing an ongoing count for arrays with metadata managed by + userspace. + + slot + This gives the role that the device has in the array. It will + either be 'none' if the device is not active in the array + (i.e. is a spare or has failed) or an integer less than the + 'raid_disks' number for the array indicating which position + it currently fills. This can only be set while assembling an + array. A device for which this is set is assumed to be working. + + offset + This gives the location in the device (in sectors from the + start) where data from the array will be stored. Any part of + the device before this offset us not touched, unless it is + used for storing metadata (Formats 1.1 and 1.2). + + size + The amount of the device, after the offset, that can be used + for storage of data. This will normally be the same as the + component_size. This can be written while assembling an + array. If a value less than the current component_size is + written, it will be rejected. + + recovery_start + + When the device is not 'in_sync', this records the number of + sectors from the start of the device which are known to be + correct. This is normally zero, but during a recovery + operation is will steadily increase, and if the recovery is + interrupted, restoring this value can cause recovery to + avoid repeating the earlier blocks. With v1.x metadata, this + value is saved and restored automatically. + + This can be set whenever the device is not an active member of + the array, either before the array is activated, or before + the 'slot' is set. + + Setting this to 'none' is equivalent to setting 'in_sync'. + Setting to any other value also clears the 'in_sync' flag. + + + +An active md device will also contain and entry for each active device +in the array. These are named + + rdNN + +where 'NN' is the position in the array, starting from 0. +So for a 3 drive array there will be rd0, rd1, rd2. +These are symbolic links to the appropriate 'dev-XXX' entry. +Thus, for example, + cat /sys/block/md*/md/rd*/state +will show 'in_sync' on every line. + + + +Active md devices for levels that support data redundancy (1,4,5,6) +also have + + sync_action + a text file that can be used to monitor and control the rebuild + process. It contains one word which can be one of: + resync - redundancy is being recalculated after unclean + shutdown or creation + recover - a hot spare is being built to replace a + failed/missing device + idle - nothing is happening + check - A full check of redundancy was requested and is + happening. This reads all block and checks + them. A repair may also happen for some raid + levels. + repair - A full check and repair is happening. This is + similar to 'resync', but was requested by the + user, and the write-intent bitmap is NOT used to + optimise the process. + + This file is writable, and each of the strings that could be + read are meaningful for writing. + + 'idle' will stop an active resync/recovery etc. There is no + guarantee that another resync/recovery may not be automatically + started again, though some event will be needed to trigger + this. + 'resync' or 'recovery' can be used to restart the + corresponding operation if it was stopped with 'idle'. + 'check' and 'repair' will start the appropriate process + providing the current state is 'idle'. + + This file responds to select/poll. Any important change in the value + triggers a poll event. Sometimes the value will briefly be + "recover" if a recovery seems to be needed, but cannot be + achieved. In that case, the transition to "recover" isn't + notified, but the transition away is. + + degraded + This contains a count of the number of devices by which the + arrays is degraded. So an optimal array with show '0'. A + single failed/missing drive will show '1', etc. + This file responds to select/poll, any increase or decrease + in the count of missing devices will trigger an event. + + mismatch_count + When performing 'check' and 'repair', and possibly when + performing 'resync', md will count the number of errors that are + found. The count in 'mismatch_cnt' is the number of sectors + that were re-written, or (for 'check') would have been + re-written. As most raid levels work in units of pages rather + than sectors, this my be larger than the number of actual errors + by a factor of the number of sectors in a page. + + bitmap_set_bits + If the array has a write-intent bitmap, then writing to this + attribute can set bits in the bitmap, indicating that a resync + would need to check the corresponding blocks. Either individual + numbers or start-end pairs can be written. Multiple numbers + can be separated by a space. + Note that the numbers are 'bit' numbers, not 'block' numbers. + They should be scaled by the bitmap_chunksize. + + sync_speed_min + sync_speed_max + This are similar to /proc/sys/dev/raid/speed_limit_{min,max} + however they only apply to the particular array. + If no value has been written to these, of if the word 'system' + is written, then the system-wide value is used. If a value, + in kibibytes-per-second is written, then it is used. + When the files are read, they show the currently active value + followed by "(local)" or "(system)" depending on whether it is + a locally set or system-wide value. + + sync_completed + This shows the number of sectors that have been completed of + whatever the current sync_action is, followed by the number of + sectors in total that could need to be processed. The two + numbers are separated by a '/' thus effectively showing one + value, a fraction of the process that is complete. + A 'select' on this attribute will return when resync completes, + when it reaches the current sync_max (below) and possibly at + other times. + + sync_max + This is a number of sectors at which point a resync/recovery + process will pause. When a resync is active, the value can + only ever be increased, never decreased. The value of 'max' + effectively disables the limit. + + + sync_speed + This shows the current actual speed, in K/sec, of the current + sync_action. It is averaged over the last 30 seconds. + + suspend_lo + suspend_hi + The two values, given as numbers of sectors, indicate a range + within the array where IO will be blocked. This is currently + only supported for raid4/5/6. + + sync_min + sync_max + The two values, given as numbers of sectors, indicate a range + within the array where 'check'/'repair' will operate. Must be + a multiple of chunk_size. When it reaches "sync_max" it will + pause, rather than complete. + You can use 'select' or 'poll' on "sync_completed" to wait for + that number to reach sync_max. Then you can either increase + "sync_max", or can write 'idle' to "sync_action". + + +Each active md device may also have attributes specific to the +personality module that manages it. +These are specific to the implementation of the module and could +change substantially if the implementation changes. + +These currently include + + stripe_cache_size (currently raid5 only) + number of entries in the stripe cache. This is writable, but + there are upper and lower limits (32768, 16). Default is 128. + strip_cache_active (currently raid5 only) + number of active entries in the stripe cache + preread_bypass_threshold (currently raid5 only) + number of times a stripe requiring preread will be bypassed by + a stripe that does not require preread. For fairness defaults + to 1. Setting this to 0 disables bypass accounting and + requires preread stripes to wait until all full-width stripe- + writes are complete. Valid values are 0 to stripe_cache_size. |