Array of Independent Devices.
.PP
.B md
-supports RAID levels 1 (mirroring) 4 (striped array with parity
-device), 5 (striped array with distributed parity information) and 6
-(striped array with distributed dual redundancy information.) If
-some number of underlying devices fails while using one of these
+supports RAID levels
+1 (mirroring),
+4 (striped array with parity device),
+5 (striped array with distributed parity information),
+6 (striped array with distributed dual redundancy information), and
+10 (striped and mirrored).
+If some number of underlying devices fails while using one of these
levels, the array will continue to function; this number is one for
RAID levels 4 and 5, two for RAID level 6, and all but one (N-1) for
-RAID level 1.
+RAID level 1, and dependant on configuration for level 10.
.PP
.B md
also supports a number of pseudo RAID (non-redundant) configurations
variations can be stored at the start of the device (version 1.1) or 4K from
the start of the device (version 1.2).
This superblock format stores multibyte data in a
-processor-independant format and has supports upto hundreds of
+processor-independent format and has supports up to hundreds of
component devices (version 0.90 only supports 28).
The superblock contains, among other things:
.TP
LEVEL
The manner in which the devices are arranged into the array
-(linear, raid0, raid1, raid4, raid5, multipath).
+(linear, raid0, raid1, raid4, raid5, raid10, multipath).
.TP
UUID
a 128 bit Universally Unique Identifier that identifies the array that
RAID1
In some configurations it might be desired to create a raid1
configuration that does use a superblock, and to maintain the state of
-the array elsewhere. While not encouraged, this is supported.
+the array elsewhere. While not encouraged for general us, it does
+have special-purpose uses and is supported.
.SS LINEAR
data that is on the array. However this cannot be done on a live
array.
+If a chunksize is given with a LINEAR array, the usable space on each
+device is rounded down to a multiple of this chunksize.
.SS RAID0
normal mode and single disk failure mode. It is very slow in dual
disk failure mode, however.
+.SS RAID10
+
+RAID10 provides a combination of RAID1 and RAID0, and sometimes known
+as RAID1+0. Every datablock is duplicated some number of times, and
+the resulting collection of datablocks are distributed over multiple
+drives.
+
+When configuring a RAID10 array it is necessary to specify the number
+of replicas of each data block that are required (this will normally
+be 2) and whether the replicas should be 'near', 'offset' or 'far'.
+(Note that the 'offset' layout is only available from 2.6.18).
+
+When 'near' replicas are chosen, the multiple copies of a given chunk
+are laid out consecutively across the stripes of the array, so the two
+copies of a datablock will likely be at the same offset on two
+adjacent devices.
+
+When 'far' replicas are chosen, the multiple copies of a given chunk
+are laid out quite distant from each other. The first copy of all
+data blocks will be striped across the early part of all drives in
+RAID0 fashion, and then the next copy of all blocks will be striped
+across a later section of all drives, always ensuring that all copies
+of any given block are on different drives.
+
+The 'far' arrangement can give sequential read performance equal to
+that of a RAID0 array, but at the cost of degraded write performance.
+
+When 'offset' replicas are chosen, the multiple copies of a given
+chunk are laid out on consecutive drives and at consecutive offsets.
+Effectively each stripe is duplicated and the copies are offset by one
+device. This should give similar read characteristics to 'far' if a
+suitably large chunk size is used, but without as much seeking for
+writes.
+
+It should be noted that the number of devices in a RAID10 array need
+not be a multiple of the number of replica of each data block, those
+there must be at least as many devices as replicas.
+
+If, for example, an array is created with 5 devices and 2 replicas,
+then space equivalent to 2.5 of the devices will be available, and
+every block will be stored on two different devices.
+
+Finally, it is possible to have an array with both 'near' and 'far'
+copies. If and array is configured with 2 near copies and 2 far
+copies, then there will be a total of 4 copies of each block, each on
+a different drive. This is an artifact of the implementation and is
+unlikely to be of real value.
+
.SS MUTIPATH
MULTIPATH is not really a RAID at all as there is only one real device
(paths) to this device, and one of these paths might fail, so there
are some similarities.
-A MULTIPATH array is composed of a number of logical different
+A MULTIPATH array is composed of a number of logically different
devices, often fibre channel interfaces, that all refer the the same
real device. If one of these interfaces fails (e.g. due to cable
-problems), the multipath driver to attempt to redirect requests to
+problems), the multipath driver will attempt to redirect requests to
another interface.
.SS FAULTY
the data in the component device.
The FAULTY module may be requested to simulate faults to allow testing
-of other md levels or of filesystem. Faults can be chosen to trigger
+of other md levels or of filesystems. Faults can be chosen to trigger
on read requests or write requests, and can be transient (a subsequent
-read/write at the address will probably succeed) or persistant
+read/write at the address will probably succeed) or persistent
(subsequent read/write of the same address will fail). Further, read
faults can be "fixable" meaning that they persist until a write
request at the same address.
Fault types can be requested with a period. In this case the fault
-will recur repeatedly after the given number of request of the
-relevant time. For example if persistent read faults have a period of
-100, then ever 100th read request would generate a fault, and the
+will recur repeatedly after the given number of requests of the
+relevant type. For example if persistent read faults have a period of
+100, then every 100th read request would generate a fault, and the
faulty sector would be recorded so that subsequent reads on that
sector would also fail.
Faults generated after this limit is exhausted are treated as
transient.
-It list of faulty sectors can be flushed, and the active list of
+The list of faulty sectors can be flushed, and the active list of
failure modes can be cleared.
.SS UNCLEAN SHUTDOWN
-When changes are made to a RAID1, RAID4, RAID5 or RAID6 array there is a
-possibility of inconsistency for short periods of time as each update
-requires are least two block to be written to different devices, and
-these writes probably wont happen at exactly the same time.
-Thus if a system with one of these arrays is shutdown in the middle of
-a write operation (e.g. due to power failure), the array may not be
-consistent.
+When changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array
+there is a possibility of inconsistency for short periods of time as
+each update requires are least two block to be written to different
+devices, and these writes probably wont happen at exactly the same
+time. Thus if a system with one of these arrays is shutdown in the
+middle of a write operation (e.g. due to power failure), the array may
+not be consistent.
To handle this situation, the md driver marks an array as "dirty"
before writing any data to it, and marks it as "clean" when the array
inconsistency. For RAID1, this involves copying the contents of the
first drive onto all other drives. For RAID4, RAID5 and RAID6 this
involves recalculating the parity for each stripe and making sure that
-the parity block has the correct data. This process, known as
-"resynchronising" or "resync" is performed in the background. The
-array can still be used, though possibly with reduced performance.
+the parity block has the correct data. For RAID10 it involves copying
+one of the replicas of each block onto all the others. This process,
+known as "resynchronising" or "resync" is performed in the background.
+The array can still be used, though possibly with reduced performance.
If a RAID4, RAID5 or RAID6 array is degraded (missing at least one
drive) when it is restarted after an unclean shutdown, it cannot
recalculate parity, and so it is possible that data might be
undetectably corrupted. The 2.4 md driver
.B does not
-alert the operator to this condition. The 2.5 md driver will fail to
-start an array in this condition without manual intervention.
+alert the operator to this condition. The 2.6 md driver will fail to
+start an array in this condition without manual intervention, though
+this behaviour can be over-ridden by a kernel parameter.
.SS RECOVERY
-If the md driver detects any error on a device in a RAID1, RAID4,
-RAID5 or RAID6 array, it immediately disables that device (marking it
-as faulty) and continues operation on the remaining devices. If there
-is a spare drive, the driver will start recreating on one of the spare
-drives the data what was on that failed drive, either by copying a
-working drive in a RAID1 configuration, or by doing calculations with
-the parity block on RAID4, RAID5 or RAID6.
+If the md driver detects a write error on a device in a RAID1, RAID4,
+RAID5, RAID6, or RAID10 array, it immediately disables that device
+(marking it as faulty) and continues operation on the remaining
+devices. If there is a spare drive, the driver will start recreating
+on one of the spare drives the data what was on that failed drive,
+either by copying a working drive in a RAID1 configuration, or by
+doing calculations with the parity block on RAID4, RAID5 or RAID6, or
+by finding a copying originals for RAID10.
+
+In kernels prior to about 2.6.15, a read error would cause the same
+effect as a write error. In later kernels, a read-error will instead
+cause md to attempt a recovery by overwriting the bad block. i.e. it
+will find the correct data from elsewhere, write it over the block
+that failed, and then try to read it back again. If either the write
+or the re-read fail, md will treat the error the same way that a write
+error is treated and will fail the whole device.
While this recovery process is happening, the md driver will monitor
accesses to the array and will slow down the rate of recovery if other
.B speed_limit_max
control files mentioned below.
+.SS BITMAP WRITE-INTENT LOGGING
+
+From Linux 2.6.13,
+.I md
+supports a bitmap based write-intent log. If configured, the bitmap
+is used to record which blocks of the array may be out of sync.
+Before any write request is honoured, md will make sure that the
+corresponding bit in the log is set. After a period of time with no
+writes to an area of the array, the corresponding bit will be cleared.
+
+This bitmap is used for two optimisations.
+
+Firstly, after an unclear shutdown, the resync process will consult
+the bitmap and only resync those blocks that correspond to bits in the
+bitmap that are set. This can dramatically increase resync time.
+
+Secondly, when a drive fails and is removed from the array, md stops
+clearing bits in the intent log. If that same drive is re-added to
+the array, md will notice and will only recover the sections of the
+drive that are covered by bits in the intent log that are set. This
+can allow a device to be temporarily removed and reinserted without
+causing an enormous recovery cost.
+
+The intent log can be stored in a file on a separate device, or it can
+be stored near the superblocks of an array which has superblocks.
+
+It is possible to add an intent log or an active array, or remove an
+intent log if one is present.
+
+In 2.6.13, intent bitmaps are only supported with RAID1. Other levels
+with redundancy are supported from 2.6.15.
+
+.SS WRITE-BEHIND
+
+From Linux 2.6.14,
+.I md
+supports WRITE-BEHIND on RAID1 arrays.
+
+This allows certain devices in the array to be flagged as
+.IR write-mostly .
+MD will only read from such devices if there is no
+other option.
+
+If a write-intent bitmap is also provided, write requests to
+write-mostly devices will be treated as write-behind requests and md
+will not wait for writes to those requests to complete before
+reporting the write as complete to the filesystem.
+
+This allows for a RAID1 with WRITE-BEHIND to be used to mirror data
+over a slow link to a remove computer (providing the link isn't too
+slow). The extra latency of the remote link will not slow down normal
+operations, but the remote system will still have a reasonably
+up-to-date copy of all data.
+
+.SS RESTRIPING
+
+.IR Restriping ,
+also known as
+.IR Reshaping ,
+is the processes of re-arranging the data stored in each stripe into a
+new layout. This might involve changing the number of devices in the
+array (so the stripes are wider) changing the chunk size (so stripes
+are deeper or shallower), or changing the arrangement of data and
+parity, possibly changing the raid level (e.g. 1 to 5 or 5 to 6).
+
+As of Linux 2.6.17, md can reshape a raid5 array to have more
+devices. Other possibilities may follow in future kernels.
+
+During any stripe process there is a 'critical section' during which
+live data is being over-written on disk. For the operation of
+increasing the number of drives in a raid5, this critical section
+covers the first few stripes (the number being the product of the old
+and new number of devices). After this critical section is passed,
+data is only written to areas of the array which no longer hold live
+data - the live data has already been located away.
+
+md is not able to ensure data preservation if there is a crash
+(e.g. power failure) during the critical section. If md is asked to
+start an array which failed during a critical section of restriping,
+it will fail to start the array.
+
+To deal with this possibility, a user-space program must
+.IP \(bu 4
+Disable writes to that section of the array (using the
+.B sysfs
+interface),
+.IP \(bu 4
+Take a copy of the data somewhere (i.e. make a backup)
+.IP \(bu 4
+Allow the process to continue and invalidate the backup and restore
+write access once the critical section is passed, and
+.IP \(bu 4
+Provide for restoring the critical data before restarting the array
+after a system crash.
+.PP
+
+.B mdadm
+version 2.4 and later will do this for growing a RAID5 array.
+
+For operations that do not change the size of the array, like simply
+increasing chunk size, or converting RAID5 to RAID6 with one extra
+device, the entire process is the critical section. In this case the
+restripe will need to progress in stages as a section is suspended,
+backed up,
+restriped, and released. This is not yet implemented.
+
+.SS SYSFS INTERFACE
+All block devices appear as a directory in
+.I sysfs
+(usually mounted at
+.BR /sys ).
+For MD devices, this directory will contain a subdirectory called
+.B md
+which contains various files for providing access to information about
+the array.
+
+This interface is documented more fully in the file
+.B Documentation/md.txt
+which is distributed with the kernel sources. That file should be
+consulted for full documentation. The following are just a selection
+of attribute files that are available.
+
+.TP
+.B md/sync_speed_min
+This value, if set, overrides the system-wide setting in
+.B /proc/sys/dev/raid/speed_limit_min
+for this array only.
+Writing the value
+.B system
+to this file cause the system-wide setting to have effect.
+
+.TP
+.B md/sync_speed_max
+This is the partner of
+.B md/sync_speed_min
+and overrides
+.B /proc/sys/dev/raid/spool_limit_max
+described below.
+
+.TP
+.B md/sync_action
+This can be used to monitor and control the resync/recovery process of
+MD.
+In particular, writing "check" here will cause the array to read all
+data block and check that they are consistent (e.g. parity is correct,
+or all mirror replicas are the same). Any discrepancies found are
+.B NOT
+corrected.
+
+A count of problems found will be stored in
+.BR md/mismatch_count .
+
+Alternately, "repair" can be written which will cause the same check
+to be performed, but any errors will be corrected.
+
+Finally, "idle" can be written to stop the check/repair process.
+
+.TP
+.B md/stripe_cache_size
+This is only available on RAID5 and RAID6. It records the size (in
+pages per device) of the stripe cache which is used for synchronising
+all read and write operations to the array. The default is 128.
+Increasing this number can increase performance in some situations, at
+some cost in system memory.
+
+
.SS KERNEL PARAMETERS
-The md driver recognised three different kernel parameters.
+The md driver recognised several different kernel parameters.
.TP
.B raid=noautodetect
This will disable the normal detection of md arrays that happens at
then if any of the 4 main partitions has a partition type of 0xFD,
then that partition will normally be inspected to see if it is part of
an MD array, and if any full arrays are found, they are started. This
-kernel paramenter disables this behaviour.
+kernel parameter disables this behaviour.
+
+.TP
+.B raid=partitionable
+.TP
+.B raid=part
+These are available in 2.6 and later kernels only. They indicate that
+autodetected MD arrays should be created as partitionable arrays, with
+a different major device number to the original non-partitionable md
+arrays. The device number is listed as
+.I mdp
+in
+.IR /proc/devices .
+
+.TP
+.B md_mod.start_ro=1
+This tells md to start all arrays in read-only mode. This is a soft
+read-only that will automatically switch to read-write on the first
+write request. However until that write request, nothing is written
+to any device by md, and in particular, no resync or recovery
+operation is started.
+
+.TP
+.B md_mod.start_dirty_degraded=1
+As mentioned above, md will not normally start a RAID4, RAID5, or
+RAID6 that is both dirty and degraded as this situation can imply
+hidden data loss. This can be awkward if the root filesystem is
+affected. Using the module parameter allows such arrays to be started
+at boot time. It should be understood that there is a real (though
+small) risk of data corruption in this situation.
.TP
.BI md= n , dev , dev ,...
+.TP
+.BI md=d n , dev , dev ,...
This tells the md driver to assemble
.B /dev/md n
from the listed devices. It is only necessary to start the device
holding the root filesystem this way. Other arrays are best started
once the system is booted.
+In 2.6 kernels, the
+.B d
+immediately after the
+.B =
+indicates that a partitionable device (e.g.
+.BR /dev/md/d0 )
+should be created rather than the original non-partitionable device.
+
.TP
.BI md= n , l , c , i , dev...
This tells the md driver to assemble a legacy RAID0 or LINEAR array
projects / thirdparty / mdadm.git / blobdiff