The
.B md
driver provides virtual devices that are created from one or more
-independant underlying devices. This array of devices often contains
+independent underlying devices. This array of devices often contains
redundancy, and hence the acronym RAID which stands for a Redundant
-Array of Independant Devices.
+Array of Independent Devices.
.PP
.B md
-support RAID levels 1 (mirroring) 4 (striped array with parity device) and 5
-(striped array with distributed parity information. If a single underlying
-device fails while using one of these level, the array will continue
-to function.
+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 a
+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.
.PP
.B md
-also supports a number of pseudo RAID (non-redundant) configuations
+also supports a number of pseudo RAID (non-redundant) configurations
including RAID0 (striped array), LINEAR (catenated array) and
MULTIPATH (a set of different interfaces to the same device).
.SS MD SUPER BLOCK
With the exception of Legacy Arrays described below, each device that
-is incorportated into an MD array has a
+is incorporated into an MD array has a
.I super block
written towards the end of the device. This superblock records
information about the structure and state of the array so that the
Early versions of the
.B md
driver only supported Linear and Raid0 configurations and so
-did not use an MD superblock (as there is not state that needs to be
+did not use an MD superblock (as there is no state that needs to be
recorded). While it is strongly recommended that all newly created
arrays utilise a superblock to help ensure that they are assembled
properly, the
array.
-
.SS RAID0
A RAID0 array (which has zero redundancy) is also known as a
striped array.
+A RAID0 array is configured at creation with a
+.B "Chunk Size"
+which must be a power of two, and at least 4 kibibytes.
+
+The RAID0 driver assigns the first chunk of the array to the first
+device, the second chunk to the second device, and so on until all
+drives have been assigned one chunk. This collection of chunks forms
+a
+.BR stripe .
+Further chunks are gathered into stripes in the same way which are
+assigned to the remaining space in the drives.
+
+If devices in the array are not all the same size, then once the
+smallest device has been exhausted, the RAID0 driver starts
+collecting chunks into smaller stripes that only span the drives which
+still have remaining space.
+
+
.SS RAID1
+
+A RAID1 array is also known as a mirrored set (though mirrors tend to
+provide reflected images, which RAID1 does not) or a plex.
+
+Once initialised, each device in a RAID1 array contains exactly the
+same data. Changes are written to all devices in parallel. Data is
+read from any one device. The driver attempts to distribute read
+requests across all devices to maximise performance.
+
+All devices in a RAID1 array should be the same size. If they are
+not, then only the amount of space available on the smallest device is
+used. Any extra space on other devices is wasted.
+
.SS RAID4
+
+A RAID4 array is like a RAID0 array with an extra device for storing
+parity. This device is the last of the active devices in the
+array. Unlike RAID0, RAID4 also requires that all stripes span all
+drives, so extra space on devices that are larger than the smallest is
+wasted.
+
+When any block in a RAID4 array is modified the parity block for that
+stripe (i.e. the block in the parity device at the same device offset
+as the stripe) is also modified so that the parity block always
+contains the "parity" for the whole stripe. i.e. its contents is
+equivalent to the result of performing an exclusive-or operation
+between all the data blocks in the stripe.
+
+This allows the array to continue to function if one device fails.
+The data that was on that device can be calculated as needed from the
+parity block and the other data blocks.
+
.SS RAID5
+
+RAID5 is very similar to RAID4. The difference is that the parity
+blocks for each stripe, instead of being on a single device, are
+distributed across all devices. This allows more parallelism when
+writing as two different block updates will quite possibly affect
+parity blocks on different devices so there is less contention.
+
+This also allows more parallelism when reading as read requests are
+distributed over all the devices in the array instead of all but one.
+
+.SS RAID6
+
+RAID6 is similar to RAID5, but can handle the loss of any \fItwo\fP
+devices without data loss. Accordingly, it requires N+2 drives to
+store N drives worth of data.
+
+The performance for RAID6 is slightly lower but comparable to RAID5 in
+normal mode and single disk failure mode. It is very slow in dual
+disk failure mode, however.
+
.SS MUTIPATH
-.SS REBUILD/RESYNC
+
+MULTIPATH is not really a RAID at all as there is only one real device
+in a MULTIPATH md array. However there are multiple access points
+(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
+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
+another interface.
+
+.SS FAULTY
+The FAULTY md module is provided for testing purposes. A faulty array
+has exactly one component device and is normally assembled without a
+superblock, so the md array created provides direct access to all of
+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
+on read requests or write requests, and can be transient (a subsequent
+read/write at the address will probably succeed) or persistant
+(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
+faulty sector would be recorded so that subsequent reads on that
+sector would also fail.
+
+There is a limit to the number of faulty sectors that are remembered.
+Faults generated after this limit is exhausted are treated as
+transient.
+
+It 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.
+
+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
+is being disabled, e.g. at shutdown. If the md driver finds an array
+to be dirty at startup, it proceeds to correct any possibly
+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.
+
+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.
+
+.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.
+
+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
+activity is happening, so that normal access to the array will not be
+unduly affected. When no other activity is happening, the recovery
+process proceeds at full speed. The actual speed targets for the two
+different situations can be controlled by the
+.B speed_limit_min
+and
+.B speed_limit_max
+control files mentioned below.
+
+.SS KERNEL PARAMETERS
+
+The md driver recognised three different kernel parameters.
+.TP
+.B raid=noautodetect
+This will disable the normal detection of md arrays that happens at
+boot time. If a drive is partitioned with MS-DOS style partitions,
+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.
+
+.TP
+.BI md= 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.
+
+.TP
+.BI md= n , l , c , i , dev...
+This tells the md driver to assemble a legacy RAID0 or LINEAR array
+without a superblock.
+.I n
+gives the md device number,
+.I l
+gives the level, 0 for RAID0 or -1 for LINEAR,
+.I c
+gives the chunk size as a base-2 logarithm offset by twelve, so 0
+means 4K, 1 means 8K.
+.I i
+is ignored (legacy support).
+
.SH FILES
.TP
.B /proc/mdstat
projects / thirdparty / mdadm.git / blobdiff