Tools that manage md devices can be found at 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=,,,,dev0,dev1,...,devn for raid arrays with persistent superblocks md=,dev0,dev1,...,devn or, to assemble a partitionable array: md=d,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 ) 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 it's 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 (not currently possible) this will contain the larger of the old and new sizes. Some raid level (RAID1) 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. chunk_size This is the size if bytes for 'chunks' and is only relevant to raid levels that involve striping (1,4,5,6,10). 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. 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. resync_start The point at which resync should start. If no resync is needed, this will be a very large number. 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. 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). 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. 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. 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, component_size will be reduced to this value. 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'. 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. 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. 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