D\bDE\bES\bSC\bCR\bRI\bIP\bPT\bTI\bIO\bON\bN
The m\bmd\bd driver provides virtual devices that are created
- from one or more independant underlying devices. This
+ from one or more independent underlying devices. This
array of devices often contains redundancy, and hence the
acronym RAID which stands for a Redundant Array of Inde-
- pendant Devices.
+ pendent Devices.
m\bmd\bd support RAID levels 1 (mirroring) 4 (striped array with
parity device) and 5 (striped array with distributed par-
function.
m\bmd\bd also supports a number of pseudo RAID (non-redundant)
- configuations including RAID0 (striped array), LINEAR
+ configurations including RAID0 (striped array), LINEAR
(catenated array) and MULTIPATH (a set of different inter-
faces to the same device).
M\bMD\bD S\bSU\bUP\bPE\bER\bR B\bBL\bLO\bOC\bCK\bK
With the exception of Legacy Arrays described below, each
- device that is incorportated into an MD array has a _\bs_\bu_\bp_\be_\br
+ device that is incorporated into an MD array has a _\bs_\bu_\bp_\be_\br
_\bb_\bl_\bo_\bc_\bk 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-
-
R\bRA\bAI\bID\bD0\b0
A RAID0 array (which has zero redundancy) is also known as
- a striped array.
+ a striped array. A RAID0 array is configured at creation
+ with a C\bCh\bhu\bun\bnk\bk S\bSi\biz\bze\be which must be a multiple of 4 kibibytes.
+
+ The RAID0 driver places 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 a s\bst\btr\bri\bip\bpe\be. 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 collecting chunks into smaller stripes that
+ only span the drives which still have remaining space.
+
+
R\bRA\bAI\bID\bD1\b1
+ A RAID1 array is also known as a mirrored set (though mir-
+ rors tend to provide reflect 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.
+
+
R\bRA\bAI\bID\bD4\b4
+ 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 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.
+
+
R\bRA\bAI\bID\bD5\b5
+ 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.
+
+
M\bMU\bUT\bTI\bIP\bPA\bAT\bTH\bH
- R\bRE\bEB\bBU\bUI\bIL\bLD\bD/\b/R\bRE\bES\bSY\bYN\bNC\bC
+ 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 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.
+
+
+
+ U\bUN\bNC\bCL\bLE\bEA\bAN\bN S\bSH\bHU\bUT\bTD\bDO\bOW\bWN\bN
+ When changes are made to an 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. This is 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" before writing any data to it, and marks it as
+ "clean" when the array is being disabled, e.g. at shut-
+ down. If the md driver finds an array to be dirty at
+ startup, it proceeds to correct any possibly inconsis-
+ tency. 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 d\bdo\boe\bes\bs
+ n\bno\bot\bt alert the operator to this condition. It should prob-
+ ably fail to start an array in this condition without man-
+ ual intervention.
+
+
+ R\bRE\bEC\bCO\bOV\bVE\bER\bRY\bY
+ 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.
+
+ Why 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 dif-
+ ferent situations can be controlled by the s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bmi\bin\bn
+ and s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bma\bax\bx control files mentioned below.
+
+
+
F\bFI\bIL\bLE\bES\bS
/\b/p\bpr\bro\boc\bc/\b/m\bmd\bds\bst\bta\bat\bt
Contains information about the status of currently
projects / thirdparty / mdadm.git / blobdiff