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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
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
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
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
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
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
MD devices in sysfs
md devices appear in sysfs (/sys) as regular block devices,
Each 'md' device will contain a subdirectory called 'md' which
contains further md-specific information about the device.
All md devices contain:
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.
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.
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
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.
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.
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'
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.
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.
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.
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.
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.
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.
No devices, no size, no level
Writing is equivalent to STOP_ARRAY ioctl
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
no resync can happen. no superblocks get written.
write requests fail
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.
fully active: IO and resync can be happening.
When written to inactive array, starts with resync
clean, but writes are blocked waiting for 'active' to be written.
like active, but no writes have been seen for a while (safe_mode_delay).
This indicates where the write-intent bitmap for the array is
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
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).
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.
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
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)
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
where XXX is a name that the kernel knows for the device, e.g. hdb1.
Each directory contains:
a symlink to the block device in /sys/block, e.g.
/sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
A file containing an image of the superblock read from, or
written to, that device.
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.
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
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.
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).
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.
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
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
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
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
'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.
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.
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.
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.
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.
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.
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.
This shows the current actual speed, in K/sec, of the current
sync_action. It is averaged over the last 30 seconds.
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
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.