diff options
Diffstat (limited to 'md.man')
| -rw-r--r-- | md.man | 137 |
1 files changed, 130 insertions, 7 deletions
@@ -11,10 +11,10 @@ SSYYNNOOPPSSIISS DDEESSCCRRIIPPTTIIOONN The mmdd 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. mmdd support RAID levels 1 (mirroring) 4 (striped array with parity device) and 5 (striped array with distributed par- @@ -23,14 +23,14 @@ DDEESSCCRRIIPPTTIIOONN function. mmdd 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). MMDD SSUUPPEERR BBLLOOCCKK With the exception of Legacy Arrays described below, each - device that is incorportated into an MD array has a _s_u_p_e_r + device that is incorporated into an MD array has a _s_u_p_e_r _b_l_o_c_k 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- @@ -77,16 +77,139 @@ DDEESSCCRRIIPPTTIIOONN - RRAAIIDD00 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 CChhuunnkk SSiizzee 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 ssttrriippee. 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. + + RRAAIIDD11 + 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. + + RRAAIIDD44 + 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. + + RRAAIIDD55 + 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. + + MMUUTTIIPPAATTHH - RREEBBUUIILLDD//RREESSYYNNCC + 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. + + + + UUNNCCLLEEAANN SSHHUUTTDDOOWWNN + 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 ddooeess + nnoott alert the operator to this condition. It should prob- + ably fail to start an array in this condition without man- + ual intervention. + + + RREECCOOVVEERRYY + 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 ssppeeeedd__lliimmiitt__mmiinn + and ssppeeeedd__lliimmiitt__mmaaxx control files mentioned below. + + + FFIILLEESS //pprroocc//mmddssttaatt Contains information about the status of currently |