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
+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) configurations
-including RAID0 (striped array), LINEAR (catenated array) and
-MULTIPATH (a set of different interfaces to the same device).
+including RAID0 (striped array), LINEAR (catenated array),
+MULTIPATH (a set of different interfaces to the same device),
+and FAULTY (a layer over a single device into which errors can be injected).
.SS MD SUPER BLOCK
-With the exception of Legacy Arrays described below, each device that
-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
-array can be reliably re-assembled after a shutdown.
-
-The superblock is 4K long and is written into a 64K aligned block that
+Each device in an array may have a
+.I superblock
+which records information about the structure and state of the array.
+This allows the array to be reliably re-assembled after a shutdown.
+
+From Linux kernel version 2.6.10,
+.B md
+provides support for two different formats of this superblock, and
+other formats can be added. Prior to this release, only one format is
+supported.
+
+The common format - known as version 0.90 - has
+a superblock that is 4K long and is written into a 64K aligned block that
starts at least 64K and less than 128K from the end of the device
(i.e. to get the address of the superblock round the size of the
device down to a multiple of 64K and then subtract 64K).
The available size of each device is the amount of space before the
super block, so between 64K and 128K is lost when a device in
incorporated into an MD array.
+This superblock stores multi-byte fields in a processor-dependant
+manner, so arrays cannot easily be moved between computers with
+different processors.
+
+The new format - known as version 1 - has a superblock that is
+normally 1K long, but can be longer. It is normally stored between 8K
+and 12K from the end of the device, on a 4K boundary, though
+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
+component devices (version 0.90 only supports 28).
The superblock contains, among other things:
.TP
a 128 bit Universally Unique Identifier that identifies the array that
this device is part of.
-.SS LEGACY ARRAYS
+.SS ARRAYS WITHOUT SUPERBLOCKS
+While it is usually best to create arrays with superblocks so that
+they can be assembled reliably, there are some circumstances where an
+array without superblocks in preferred. This include:
+.TP
+LEGACY ARRAYS
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
-recorded). While it is strongly recommended that all newly created
-arrays utilise a superblock to help ensure that they are assembled
-properly, the
+driver only supported Linear and Raid0 configurations and did not use
+a superblock (which is less critical with these configurations).
+While such arrays should be rebuilt with superblocks if possible,
.B md
-driver still supports legacy linear and raid0 md arrays that
-do not have a superblock.
+continues to support them.
+.TP
+FAULTY
+Being a largely transparent layer over a different device, the FAULTY
+personality doesn't gain anything from having a superblock.
+.TP
+MULTIPATH
+It is often possible to detect devices which are different paths to
+the same storage directly rather than having a distinctive superblock
+written to the device and searched for on all paths. In this case,
+a MULTIPATH array with no superblock makes sense.
+.TP
+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.
.SS LINEAR
striped array.
A RAID0 array is configured at creation with a
.B "Chunk Size"
-which must be a multiple of 4 kibibytes.
+which must be a power of two, and at least 4 kibibytes.
-The RAID0 driver places the first chunk of the array to the first
+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 chuck. This collection of chunks forms
+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 device in the array are not all the same size, then once the
-smallest devices has been exhausted, the RAID0 driver starts
+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 reflect images, which RAID1 does not) or a plex.
+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
.SS RAID4
A RAID4 array is like a RAID0 array with an extra device for storing
-parity. Unlike RAID0, RAID4 also requires that all stripes span all
+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.
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
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 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.
-
+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 an RAID1, RAID4, or RAID5 array there is a
+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.
-This is a system with one of these arrays is shutdown in the middle of
+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.
-The handle this situation, the md driver marks an array as "dirty"
+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 or RAID5 this involves recalculating the parity for each
-stripe and making sure that the parity block has the correct data.
-
-If a RAID4 or RAID5 array is degraded (missing 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 md driver currently
+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. It should probably fail to
+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, or
-RAID5 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 and RAID5.
+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.
-Why this recovery process is happening, the md driver will monitor
+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
.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
projects / thirdparty / mdadm.git / blobdiff