Array of Independent Devices.
.PP
.B md
-supports 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 levels,
-the array will continue to function.
+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, and dependant of configuration for level 10.
.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
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
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 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
+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
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
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 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' or 'far'.
+
+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.
+
+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 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 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
+(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 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.
+
+There is a limit to the number of faulty sectors that are remembered.
+Faults generated after this limit is exhausted are treated as
+transient.
+
+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, or RAID5 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
-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.
-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 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 2.4 md driver
+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. 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.
.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, 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.
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.
+
+Subsequent versions of Linux will support hot-adding of bitmaps to
+existing arrays.
+
+In 2.6.13, intent bitmaps are only supported with RAID1. Other levels
+will follow.
+
+.SS WRITE-BEHIND
+
+From Linux 2.6.14,
+.I md
+will support 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 KERNEL PARAMETERS
The md driver recognised three different kernel parameters.
an MD array, and if any full arrays are found, they are started. This
kernel paramenter 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
.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