-MD(4) MD(4)
-
-
-
-N\bNA\bAM\bME\bE
- md - Multiple Device driver aka Linux Software Raid
-
-S\bSY\bYN\bNO\bOP\bPS\bSI\bIS\bS
- /\b/d\bde\bev\bv/\b/m\bmd\bd_\bn
- /\b/d\bde\bev\bv/\b/m\bmd\bd/\b/_\bn
-
-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 independent underlying devices. This
- array of devices often contains redundancy, and hence the
- acronym RAID which stands for a Redundant Array of Inde-
- pendent Devices.
-
- m\bmd\bd support RAID levels 1 (mirroring) 4 (striped array with
- parity device) and 5 (striped array with distributed par-
- ity information. If a single underlying device fails
- while using one of these level, the array will continue to
- function.
-
- m\bmd\bd also supports a number of pseudo RAID (non-redundant)
- 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 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-
- assembled after a shutdown.
-
- The superblock 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.
-
- The superblock contains, among other things:
-
- LEVEL The manner in which the devices are arranged into
- the array (linear, raid0, raid1, raid4, raid5, mul-
- tipath).
-
- UUID a 128 bit Universally Unique Identifier that iden-
- tifies the array that this device is part of.
-
-
- L\bLE\bEG\bGA\bAC\bCY\bY A\bAR\bRR\bRA\bAY\bYS\bS
- Early versions of the m\bmd\bd 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 assem-
- bled properly, the m\bmd\bd driver still supports legacy linear
- and raid0 md arrays that do not have a superblock.
-
-
- L\bLI\bIN\bNE\bEA\bAR\bR
- A linear array simply catenates the available space on
- each drive together to form one large virtual drive.
-
- One advantage of this arrangement over the more common
- RAID0 arrangement is that the array may be reconfigured at
- a later time with an extra drive and so the array is made
- bigger without disturbing the data that is on the array.
- However this cannot be done on a live array.
-
-
-
- R\bRA\bAI\bID\bD0\b0
- A RAID0 array (which has zero redundancy) is also known as
- 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
- 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
- running array.
-
- /\b/p\bpr\bro\boc\bc/\b/s\bsy\bys\bs/\b/d\bde\bev\bv/\b/r\bra\bai\bid\bd/\b/s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bmi\bin\bn
- A readable and writable file that reflects the cur-
- rent goal rebuild speed for times when non-rebuild
- activity is current on an array. The speed is in
- Kibibytes per second, and is a per-device rate, not
- a per-array rate (which means that an array with
- more disc will shuffle more data for a given
- speed). The default is 100.
-
-
- /\b/p\bpr\bro\boc\bc/\b/s\bsy\bys\bs/\b/d\bde\bev\bv/\b/r\bra\bai\bid\bd/\b/s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bma\bax\bx
- A readable and writable file that reflects the cur-
- rent goal rebuild speed for times when no non-
- rebuild activity is current on an array. The
- default is 100,000.
-
-
-S\bSE\bEE\bE A\bAL\bLS\bSO\bO
- m\bmd\bda\bad\bdm\bm(8), m\bmk\bkr\bra\bai\bid\bd(8).
-
-
-
- MD(4)
projects / thirdparty / mdadm.git / commitdiff
