md.4

   1 .TH MD 4
   2 .SH NAME
   3 md \- Multiple Device driver aka Linux Software Raid
   4 .SH SYNOPSIS
   5 .BI /dev/md n
   6 .br
   7 .BI /dev/md/ n
   8 .SH DESCRIPTION
   9 The
  10 .B md
  11 driver provides virtual devices that are created from one or more
  12 independent underlying devices.  This array of devices often contains
  13 redundancy, and hence the acronym RAID which stands for a Redundant
  14 Array of Independent Devices.
  15 .PP
  16 .B md
  17 supports RAID levels 1 (mirroring) 4 (striped array with parity
  18 device), 5 (striped array with distributed parity information) and 6
  19 (striped array with distributed dual redundancy information.)  If
  20 some number of underlying devices fails while using one of these
  21 levels, the array will continue to function; this number is one for
  22 RAID levels 4 and 5, two for RAID level 6, and all but one (N-1) for
  23 RAID level 1.
  24 .PP
  25 .B md
  26 also supports a number of pseudo RAID (non-redundant) configurations
  27 including RAID0 (striped array), LINEAR (catenated array),
  28 MULTIPATH (a set of different interfaces to the same device),
  29 and FAULTY (a layer over a single device into which errors can be injected).
  30
  31 .SS MD SUPER BLOCK
  32 Each device in an array may have a
  33 .I superblock
  34 which records information about the structure and state of the array.
  35 This allows the array to be reliably re-assembled after a shutdown.
  36
  37 From Linux kernel version 2.6.10,
  38 .B md
  39 provides support for two different formats of this superblock, and
  40 other formats can be added.  Prior to this release, only one format is
  41 supported.
  42
  43 The common format - known as version 0.90 - has
  44 a superblock that is 4K long and is written into a 64K aligned block that
  45 starts at least 64K and less than 128K from the end of the device
  46 (i.e. to get the address of the superblock round the size of the
  47 device down to a multiple of 64K and then subtract 64K).
  48 The available size of each device is the amount of space before the
  49 super block, so between 64K and 128K is lost when a device in
  50 incorporated into an MD array.
  51 This superblock stores multi-byte fields in a processor-dependant
  52 manner, so arrays cannot easily be moved between computers with
  53 different processors.
  54
  55 The new format - known as version 1 - has a superblock that is
  56 normally 1K long, but can be longer.  It is normally stored between 8K
  57 and 12K from the end of the device, on a 4K boundary, though
  58 variations can be stored at the start of the device (version 1.1) or 4K from
  59 the start of the device (version 1.2).
  60 This superblock format stores multibyte data in a
  61 processor-independant format and has supports upto hundreds of
  62 component devices (version 0.90 only supports 28).
  63
  64 The superblock contains, among other things:
  65 .TP
  66 LEVEL
  67 The manner in which the devices are arranged into the array
  68 (linear, raid0, raid1, raid4, raid5, multipath).
  69 .TP
  70 UUID
  71 a 128 bit Universally Unique Identifier that identifies the array that
  72 this device is part of.
  73
  74 .SS ARRAYS WITHOUT SUPERBLOCKS
  75 While it is usually best to create arrays with superblocks so that
  76 they can be assembled reliably, there are some circumstances where an
  77 array without superblocks in preferred.  This include:
  78 .TP
  79 LEGACY ARRAYS
  80 Early versions of the
  81 .B md
  82 driver only supported Linear and Raid0 configurations and did not use
  83 a superblock (which is less critical with these configurations).
  84 While such arrays should be rebuilt with superblocks if possible,
  85 .B md
  86 continues to support them.
  87 .TP
  88 FAULTY
  89 Being a largely transparent layer over a different device, the FAULTY
  90 personality doesn't gain anything from having a superblock.
  91 .TP
  92 MULTIPATH
  93 It is often possible to detect devices which are different paths to
  94 the same storage directly rather than having a distinctive superblock
  95 written to the device and searched for on all paths.  In this case,
  96 a MULTIPATH array with no superblock makes sense.
  97 .TP
  98 RAID1
  99 In some configurations it might be desired to create a raid1
 100 configuration that does use a superblock, and to maintain the state of
 101 the array elsewhere.  While not encouraged, this is supported.
 102
 103 .SS LINEAR
 104
 105 A linear array simply catenates the available space on each
 106 drive together to form one large virtual drive.
 107
 108 One advantage of this arrangement over the more common RAID0
 109 arrangement is that the array may be reconfigured at a later time with
 110 an extra drive and so the array is made bigger without disturbing the
 111 data that is on the array.  However this cannot be done on a live
 112 array.
 113
 114
 115 .SS RAID0
 116
 117 A RAID0 array (which has zero redundancy) is also known as a
 118 striped array.
 119 A RAID0 array is configured at creation with a
 120 .B "Chunk Size"
 121 which must be a power of two, and at least 4 kibibytes.
 122
 123 The RAID0 driver assigns the first chunk of the array to the first
 124 device, the second chunk to the second device, and so on until all
 125 drives have been assigned one chunk.  This collection of chunks forms
 126 a
 127 .BR stripe .
 128 Further chunks are gathered into stripes in the same way which are
 129 assigned to the remaining space in the drives.
 130
 131 If devices in the array are not all the same size, then once the
 132 smallest device has been exhausted, the RAID0 driver starts
 133 collecting chunks into smaller stripes that only span the drives which
 134 still have remaining space.
 135
 136
 137 .SS RAID1
 138
 139 A RAID1 array is also known as a mirrored set (though mirrors tend to
 140 provide reflected images, which RAID1 does not) or a plex.
 141
 142 Once initialised, each device in a RAID1 array contains exactly the
 143 same data.  Changes are written to all devices in parallel.  Data is
 144 read from any one device.  The driver attempts to distribute read
 145 requests across all devices to maximise performance.
 146
 147 All devices in a RAID1 array should be the same size.  If they are
 148 not, then only the amount of space available on the smallest device is
 149 used.  Any extra space on other devices is wasted.
 150
 151 .SS RAID4
 152
 153 A RAID4 array is like a RAID0 array with an extra device for storing
 154 parity. This device is the last of the active devices in the
 155 array. Unlike RAID0, RAID4 also requires that all stripes span all
 156 drives, so extra space on devices that are larger than the smallest is
 157 wasted.
 158
 159 When any block in a RAID4 array is modified the parity block for that
 160 stripe (i.e. the block in the parity device at the same device offset
 161 as the stripe) is also modified so that the parity block always
 162 contains the "parity" for the whole stripe.  i.e. its contents is
 163 equivalent to the result of performing an exclusive-or operation
 164 between all the data blocks in the stripe.
 165
 166 This allows the array to continue to function if one device fails.
 167 The data that was on that device can be calculated as needed from the
 168 parity block and the other data blocks.
 169
 170 .SS RAID5
 171
 172 RAID5 is very similar to RAID4.  The difference is that the parity
 173 blocks for each stripe, instead of being on a single device, are
 174 distributed across all devices.  This allows more parallelism when
 175 writing as two different block updates will quite possibly affect
 176 parity blocks on different devices so there is less contention.
 177
 178 This also allows more parallelism when reading as read requests are
 179 distributed over all the devices in the array instead of all but one.
 180
 181 .SS RAID6
 182
 183 RAID6 is similar to RAID5, but can handle the loss of any \fItwo\fP
 184 devices without data loss.  Accordingly, it requires N+2 drives to
 185 store N drives worth of data.
 186
 187 The performance for RAID6 is slightly lower but comparable to RAID5 in
 188 normal mode and single disk failure mode.  It is very slow in dual
 189 disk failure mode, however.
 190
 191 .SS MUTIPATH
 192
 193 MULTIPATH is not really a RAID at all as there is only one real device
 194 in a MULTIPATH md array.  However there are multiple access points
 195 (paths) to this device, and one of these paths might fail, so there
 196 are some similarities.
 197
 198 A MULTIPATH array is composed of a number of logical different
 199 devices, often fibre channel interfaces, that all refer the the same
 200 real device. If one of these interfaces fails (e.g. due to cable
 201 problems), the multipath driver to attempt to redirect requests to
 202 another interface.
 203
 204 .SS FAULTY
 205 The FAULTY md module is provided for testing purposes.  A faulty array
 206 has exactly one component device and is normally assembled without a
 207 superblock, so the md array created provides direct access to all of
 208 the data in the component device.
 209
 210 The FAULTY module may be requested to simulate faults to allow testing
 211 of other md levels or of filesystem.  Faults can be chosen to trigger
 212 on read requests or write requests, and can be transient (a subsequent
 213 read/write at the address will probably succeed) or persistant
 214 (subsequent read/write of the same address will fail).  Further, read
 215 faults can be "fixable" meaning that they persist until a write
 216 request at the same address.
 217
 218 Fault types can be requested with a period.  In this case the fault
 219 will recur repeatedly after the given number of request of the
 220 relevant time.  For example if persistent read faults have a period of
 221 100, then ever 100th read request would generate a fault, and the
 222 faulty sector would be recorded so that subsequent reads on that
 223 sector would also fail.
 224
 225 There is a limit to the number of faulty sectors that are remembered.
 226 Faults generated after this limit is exhausted are treated as
 227 transient.
 228
 229 It list of faulty sectors can be flushed, and the active list of
 230 failure modes can be cleared.
 231
 232 .SS UNCLEAN SHUTDOWN
 233
 234 When changes are made to a RAID1, RAID4, RAID5 or RAID6 array there is a
 235 possibility of inconsistency for short periods of time as each update
 236 requires are least two block to be written to different devices, and
 237 these writes probably wont happen at exactly the same time.
 238 Thus if a system with one of these arrays is shutdown in the middle of
 239 a write operation (e.g. due to power failure), the array may not be
 240 consistent.
 241
 242 To handle this situation, the md driver marks an array as "dirty"
 243 before writing any data to it, and marks it as "clean" when the array
 244 is being disabled, e.g. at shutdown.  If the md driver finds an array
 245 to be dirty at startup, it proceeds to correct any possibly
 246 inconsistency.  For RAID1, this involves copying the contents of the
 247 first drive onto all other drives.  For RAID4, RAID5 and RAID6 this
 248 involves recalculating the parity for each stripe and making sure that
 249 the parity block has the correct data.  This process, known as
 250 "resynchronising" or "resync" is performed in the background.  The
 251 array can still be used, though possibly with reduced performance.
 252
 253 If a RAID4, RAID5 or RAID6 array is degraded (missing at least one
 254 drive) when it is restarted after an unclean shutdown, it cannot
 255 recalculate parity, and so it is possible that data might be
 256 undetectably corrupted.  The 2.4 md driver
 257 .B does not
 258 alert the operator to this condition.  The 2.5 md driver will fail to
 259 start an array in this condition without manual intervention.
 260
 261 .SS RECOVERY
 262
 263 If the md driver detects any error on a device in a RAID1, RAID4,
 264 RAID5 or RAID6 array, it immediately disables that device (marking it
 265 as faulty) and continues operation on the remaining devices.  If there
 266 is a spare drive, the driver will start recreating on one of the spare
 267 drives the data what was on that failed drive, either by copying a
 268 working drive in a RAID1 configuration, or by doing calculations with
 269 the parity block on RAID4, RAID5 or RAID6.
 270
 271 While this recovery process is happening, the md driver will monitor
 272 accesses to the array and will slow down the rate of recovery if other
 273 activity is happening, so that normal access to the array will not be
 274 unduly affected.  When no other activity is happening, the recovery
 275 process proceeds at full speed.  The actual speed targets for the two
 276 different situations can be controlled by the
 277 .B speed_limit_min
 278 and
 279 .B speed_limit_max
 280 control files mentioned below.
 281
 282 .SS KERNEL PARAMETERS
 283
 284 The md driver recognised three different kernel parameters.
 285 .TP
 286 .B raid=noautodetect
 287 This will disable the normal detection of md arrays that happens at
 288 boot time.  If a drive is partitioned with MS-DOS style partitions,
 289 then if any of the 4 main partitions has a partition type of 0xFD,
 290 then that partition will normally be inspected to see if it is part of
 291 an MD array, and if any full arrays are found, they are started.  This
 292 kernel paramenter disables this behaviour.
 293
 294 .TP
 295 .BI md= n , dev , dev ,...
 296 This tells the md driver to assemble
 297 .B /dev/md n
 298 from the listed devices.  It is only necessary to start the device
 299 holding the root filesystem this way.  Other arrays are best started
 300 once the system is booted.
 301
 302 .TP
 303 .BI md= n , l , c , i , dev...
 304 This tells the md driver to assemble a legacy RAID0 or LINEAR array
 305 without a superblock.
 306 .I n
 307 gives the md device number,
 308 .I l
 309 gives the level, 0 for RAID0 or -1 for LINEAR,
 310 .I c
 311 gives the chunk size as a base-2 logarithm offset by twelve, so 0
 312 means 4K, 1 means 8K.
 313 .I i
 314 is ignored (legacy support).
 315
 316 .SH FILES
 317 .TP
 318 .B /proc/mdstat
 319 Contains information about the status of currently running array.
 320 .TP
 321 .B /proc/sys/dev/raid/speed_limit_min
 322 A readable and writable file that reflects the current goal rebuild
 323 speed for times when non-rebuild activity is current on an array.
 324 The speed is in Kibibytes per second, and is a per-device rate, not a
 325 per-array rate (which means that an array with more disc will shuffle
 326 more data for a given speed).   The default is 100.
 327
 328 .TP
 329 .B /proc/sys/dev/raid/speed_limit_max
 330 A readable and writable file that reflects the current goal rebuild
 331 speed for times when no non-rebuild activity is current on an array.
 332 The default is 100,000.
 333
 334 .SH SEE ALSO
 335 .BR mdadm (8),
 336 .BR mkraid (8).