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Commit | Line | Data |
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56eb10c0 NB |
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 | |
e0d19036 | 12 | independent underlying devices. This array of devices often contains |
56eb10c0 | 13 | redundancy, and hence the acronym RAID which stands for a Redundant |
e0d19036 | 14 | Array of Independent Devices. |
56eb10c0 NB |
15 | .PP |
16 | .B md | |
599e5a36 NB |
17 | supports RAID levels |
18 | 1 (mirroring), | |
19 | 4 (striped array with parity device), | |
20 | 5 (striped array with distributed parity information), | |
21 | 6 (striped array with distributed dual redundancy information), and | |
22 | 10 (striped and mirrored). | |
23 | If some number of underlying devices fails while using one of these | |
98c6faba NB |
24 | levels, the array will continue to function; this number is one for |
25 | RAID levels 4 and 5, two for RAID level 6, and all but one (N-1) for | |
addc80c4 | 26 | RAID level 1, and dependant on configuration for level 10. |
56eb10c0 NB |
27 | .PP |
28 | .B md | |
e0d19036 | 29 | also supports a number of pseudo RAID (non-redundant) configurations |
570c0542 NB |
30 | including RAID0 (striped array), LINEAR (catenated array), |
31 | MULTIPATH (a set of different interfaces to the same device), | |
32 | and FAULTY (a layer over a single device into which errors can be injected). | |
56eb10c0 | 33 | |
11a3e71d | 34 | .SS MD SUPER BLOCK |
570c0542 NB |
35 | Each device in an array may have a |
36 | .I superblock | |
37 | which records information about the structure and state of the array. | |
38 | This allows the array to be reliably re-assembled after a shutdown. | |
56eb10c0 | 39 | |
570c0542 NB |
40 | From Linux kernel version 2.6.10, |
41 | .B md | |
42 | provides support for two different formats of this superblock, and | |
43 | other formats can be added. Prior to this release, only one format is | |
44 | supported. | |
45 | ||
46 | The common format - known as version 0.90 - has | |
47 | a superblock that is 4K long and is written into a 64K aligned block that | |
11a3e71d | 48 | starts at least 64K and less than 128K from the end of the device |
56eb10c0 NB |
49 | (i.e. to get the address of the superblock round the size of the |
50 | device down to a multiple of 64K and then subtract 64K). | |
11a3e71d | 51 | The available size of each device is the amount of space before the |
56eb10c0 NB |
52 | super block, so between 64K and 128K is lost when a device in |
53 | incorporated into an MD array. | |
570c0542 NB |
54 | This superblock stores multi-byte fields in a processor-dependant |
55 | manner, so arrays cannot easily be moved between computers with | |
56 | different processors. | |
57 | ||
58 | The new format - known as version 1 - has a superblock that is | |
59 | normally 1K long, but can be longer. It is normally stored between 8K | |
60 | and 12K from the end of the device, on a 4K boundary, though | |
61 | variations can be stored at the start of the device (version 1.1) or 4K from | |
62 | the start of the device (version 1.2). | |
63 | This superblock format stores multibyte data in a | |
addc80c4 | 64 | processor-independent format and has supports up to hundreds of |
570c0542 | 65 | component devices (version 0.90 only supports 28). |
56eb10c0 NB |
66 | |
67 | The superblock contains, among other things: | |
68 | .TP | |
69 | LEVEL | |
11a3e71d | 70 | The manner in which the devices are arranged into the array |
599e5a36 | 71 | (linear, raid0, raid1, raid4, raid5, raid10, multipath). |
56eb10c0 NB |
72 | .TP |
73 | UUID | |
74 | a 128 bit Universally Unique Identifier that identifies the array that | |
75 | this device is part of. | |
76 | ||
570c0542 NB |
77 | .SS ARRAYS WITHOUT SUPERBLOCKS |
78 | While it is usually best to create arrays with superblocks so that | |
79 | they can be assembled reliably, there are some circumstances where an | |
80 | array without superblocks in preferred. This include: | |
81 | .TP | |
82 | LEGACY ARRAYS | |
11a3e71d NB |
83 | Early versions of the |
84 | .B md | |
570c0542 NB |
85 | driver only supported Linear and Raid0 configurations and did not use |
86 | a superblock (which is less critical with these configurations). | |
87 | While such arrays should be rebuilt with superblocks if possible, | |
11a3e71d | 88 | .B md |
570c0542 NB |
89 | continues to support them. |
90 | .TP | |
91 | FAULTY | |
92 | Being a largely transparent layer over a different device, the FAULTY | |
93 | personality doesn't gain anything from having a superblock. | |
94 | .TP | |
95 | MULTIPATH | |
96 | It is often possible to detect devices which are different paths to | |
97 | the same storage directly rather than having a distinctive superblock | |
98 | written to the device and searched for on all paths. In this case, | |
99 | a MULTIPATH array with no superblock makes sense. | |
100 | .TP | |
101 | RAID1 | |
102 | In some configurations it might be desired to create a raid1 | |
103 | configuration that does use a superblock, and to maintain the state of | |
addc80c4 NB |
104 | the array elsewhere. While not encouraged for general us, it does |
105 | have special-purpose uses and is supported. | |
11a3e71d | 106 | |
56eb10c0 | 107 | .SS LINEAR |
11a3e71d NB |
108 | |
109 | A linear array simply catenates the available space on each | |
110 | drive together to form one large virtual drive. | |
111 | ||
112 | One advantage of this arrangement over the more common RAID0 | |
113 | arrangement is that the array may be reconfigured at a later time with | |
114 | an extra drive and so the array is made bigger without disturbing the | |
115 | data that is on the array. However this cannot be done on a live | |
116 | array. | |
117 | ||
599e5a36 NB |
118 | If a chunksize is given with a LINEAR array, the usable space on each |
119 | device is rounded down to a multiple of this chunksize. | |
11a3e71d | 120 | |
56eb10c0 | 121 | .SS RAID0 |
11a3e71d NB |
122 | |
123 | A RAID0 array (which has zero redundancy) is also known as a | |
124 | striped array. | |
e0d19036 NB |
125 | A RAID0 array is configured at creation with a |
126 | .B "Chunk Size" | |
c913b90e | 127 | which must be a power of two, and at least 4 kibibytes. |
e0d19036 | 128 | |
2d465520 | 129 | The RAID0 driver assigns the first chunk of the array to the first |
e0d19036 | 130 | device, the second chunk to the second device, and so on until all |
2d465520 | 131 | drives have been assigned one chunk. This collection of chunks forms |
e0d19036 NB |
132 | a |
133 | .BR stripe . | |
134 | Further chunks are gathered into stripes in the same way which are | |
135 | assigned to the remaining space in the drives. | |
136 | ||
2d465520 NB |
137 | If devices in the array are not all the same size, then once the |
138 | smallest device has been exhausted, the RAID0 driver starts | |
e0d19036 NB |
139 | collecting chunks into smaller stripes that only span the drives which |
140 | still have remaining space. | |
141 | ||
142 | ||
56eb10c0 | 143 | .SS RAID1 |
e0d19036 NB |
144 | |
145 | A RAID1 array is also known as a mirrored set (though mirrors tend to | |
5787fa49 | 146 | provide reflected images, which RAID1 does not) or a plex. |
e0d19036 NB |
147 | |
148 | Once initialised, each device in a RAID1 array contains exactly the | |
149 | same data. Changes are written to all devices in parallel. Data is | |
150 | read from any one device. The driver attempts to distribute read | |
151 | requests across all devices to maximise performance. | |
152 | ||
153 | All devices in a RAID1 array should be the same size. If they are | |
154 | not, then only the amount of space available on the smallest device is | |
155 | used. Any extra space on other devices is wasted. | |
156 | ||
56eb10c0 | 157 | .SS RAID4 |
e0d19036 NB |
158 | |
159 | A RAID4 array is like a RAID0 array with an extra device for storing | |
aa88f531 NB |
160 | parity. This device is the last of the active devices in the |
161 | array. Unlike RAID0, RAID4 also requires that all stripes span all | |
e0d19036 NB |
162 | drives, so extra space on devices that are larger than the smallest is |
163 | wasted. | |
164 | ||
165 | When any block in a RAID4 array is modified the parity block for that | |
166 | stripe (i.e. the block in the parity device at the same device offset | |
167 | as the stripe) is also modified so that the parity block always | |
168 | contains the "parity" for the whole stripe. i.e. its contents is | |
169 | equivalent to the result of performing an exclusive-or operation | |
170 | between all the data blocks in the stripe. | |
171 | ||
172 | This allows the array to continue to function if one device fails. | |
173 | The data that was on that device can be calculated as needed from the | |
174 | parity block and the other data blocks. | |
175 | ||
56eb10c0 | 176 | .SS RAID5 |
e0d19036 NB |
177 | |
178 | RAID5 is very similar to RAID4. The difference is that the parity | |
179 | blocks for each stripe, instead of being on a single device, are | |
180 | distributed across all devices. This allows more parallelism when | |
181 | writing as two different block updates will quite possibly affect | |
182 | parity blocks on different devices so there is less contention. | |
183 | ||
184 | This also allows more parallelism when reading as read requests are | |
185 | distributed over all the devices in the array instead of all but one. | |
186 | ||
98c6faba NB |
187 | .SS RAID6 |
188 | ||
189 | RAID6 is similar to RAID5, but can handle the loss of any \fItwo\fP | |
190 | devices without data loss. Accordingly, it requires N+2 drives to | |
191 | store N drives worth of data. | |
192 | ||
193 | The performance for RAID6 is slightly lower but comparable to RAID5 in | |
194 | normal mode and single disk failure mode. It is very slow in dual | |
195 | disk failure mode, however. | |
196 | ||
599e5a36 NB |
197 | .SS RAID10 |
198 | ||
199 | RAID10 provides a combination of RAID1 and RAID0, and sometimes known | |
200 | as RAID1+0. Every datablock is duplicated some number of times, and | |
201 | the resulting collection of datablocks are distributed over multiple | |
202 | drives. | |
203 | ||
204 | When configuring a RAID10 array it is necessary to specify the number | |
205 | of replicas of each data block that are required (this will normally | |
206 | be 2) and whether the replicas should be 'near' or 'far'. | |
207 | ||
208 | When 'near' replicas are chosen, the multiple copies of a given chunk | |
209 | are laid out consecutively across the stripes of the array, so the two | |
210 | copies of a datablock will likely be at the same offset on two | |
211 | adjacent devices. | |
212 | ||
213 | When 'far' replicas are chosen, the multiple copies of a given chunk | |
214 | are laid out quite distant from each other. The first copy of all | |
215 | data blocks will be striped across the early part of all drives in | |
216 | RAID0 fashion, and then the next copy of all blocks will be striped | |
217 | across a later section of all drives, always ensuring that all copies | |
218 | of any given block are on different drives. | |
219 | ||
220 | The 'far' arrangement can give sequential read performance equal to | |
221 | that of a RAID0 array, but at the cost of degraded write performance. | |
222 | ||
223 | It should be noted that the number of devices in a RAID10 array need | |
224 | not be a multiple of the number of replica of each data block, those | |
225 | there must be at least as many devices as replicas. | |
226 | ||
227 | If, for example, an array is created with 5 devices and 2 replicas, | |
228 | then space equivalent to 2.5 of the devices will be available, and | |
229 | every block will be stored on two different devices. | |
230 | ||
231 | Finally, it is possible to have an array with both 'near' and 'far' | |
232 | copies. If and array is configured with 2 near copies and 2 far | |
233 | copies, then there will be a total of 4 copies of each block, each on | |
234 | a different drive. This is an artifact of the implementation and is | |
235 | unlikely to be of real value. | |
236 | ||
11a3e71d | 237 | .SS MUTIPATH |
e0d19036 NB |
238 | |
239 | MULTIPATH is not really a RAID at all as there is only one real device | |
240 | in a MULTIPATH md array. However there are multiple access points | |
241 | (paths) to this device, and one of these paths might fail, so there | |
242 | are some similarities. | |
243 | ||
a9d69660 | 244 | A MULTIPATH array is composed of a number of logically different |
2d465520 NB |
245 | devices, often fibre channel interfaces, that all refer the the same |
246 | real device. If one of these interfaces fails (e.g. due to cable | |
a9d69660 | 247 | problems), the multipath driver will attempt to redirect requests to |
2d465520 | 248 | another interface. |
e0d19036 | 249 | |
b5e64645 NB |
250 | .SS FAULTY |
251 | The FAULTY md module is provided for testing purposes. A faulty array | |
252 | has exactly one component device and is normally assembled without a | |
253 | superblock, so the md array created provides direct access to all of | |
254 | the data in the component device. | |
255 | ||
256 | The FAULTY module may be requested to simulate faults to allow testing | |
a9d69660 | 257 | of other md levels or of filesystems. Faults can be chosen to trigger |
b5e64645 | 258 | on read requests or write requests, and can be transient (a subsequent |
addc80c4 | 259 | read/write at the address will probably succeed) or persistent |
b5e64645 NB |
260 | (subsequent read/write of the same address will fail). Further, read |
261 | faults can be "fixable" meaning that they persist until a write | |
262 | request at the same address. | |
263 | ||
264 | Fault types can be requested with a period. In this case the fault | |
a9d69660 NB |
265 | will recur repeatedly after the given number of requests of the |
266 | relevant type. For example if persistent read faults have a period of | |
267 | 100, then every 100th read request would generate a fault, and the | |
b5e64645 NB |
268 | faulty sector would be recorded so that subsequent reads on that |
269 | sector would also fail. | |
270 | ||
271 | There is a limit to the number of faulty sectors that are remembered. | |
272 | Faults generated after this limit is exhausted are treated as | |
273 | transient. | |
274 | ||
a9d69660 | 275 | The list of faulty sectors can be flushed, and the active list of |
b5e64645 | 276 | failure modes can be cleared. |
e0d19036 NB |
277 | |
278 | .SS UNCLEAN SHUTDOWN | |
279 | ||
599e5a36 NB |
280 | When changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array |
281 | there is a possibility of inconsistency for short periods of time as | |
282 | each update requires are least two block to be written to different | |
283 | devices, and these writes probably wont happen at exactly the same | |
284 | time. Thus if a system with one of these arrays is shutdown in the | |
285 | middle of a write operation (e.g. due to power failure), the array may | |
286 | not be consistent. | |
e0d19036 | 287 | |
2d465520 | 288 | To handle this situation, the md driver marks an array as "dirty" |
e0d19036 | 289 | before writing any data to it, and marks it as "clean" when the array |
98c6faba NB |
290 | is being disabled, e.g. at shutdown. If the md driver finds an array |
291 | to be dirty at startup, it proceeds to correct any possibly | |
292 | inconsistency. For RAID1, this involves copying the contents of the | |
293 | first drive onto all other drives. For RAID4, RAID5 and RAID6 this | |
294 | involves recalculating the parity for each stripe and making sure that | |
599e5a36 NB |
295 | the parity block has the correct data. For RAID10 it involves copying |
296 | one of the replicas of each block onto all the others. This process, | |
297 | known as "resynchronising" or "resync" is performed in the background. | |
298 | The array can still be used, though possibly with reduced performance. | |
98c6faba NB |
299 | |
300 | If a RAID4, RAID5 or RAID6 array is degraded (missing at least one | |
301 | drive) when it is restarted after an unclean shutdown, it cannot | |
302 | recalculate parity, and so it is possible that data might be | |
303 | undetectably corrupted. The 2.4 md driver | |
e0d19036 | 304 | .B does not |
addc80c4 NB |
305 | alert the operator to this condition. The 2.6 md driver will fail to |
306 | start an array in this condition without manual intervention, though | |
307 | this behaviour can be over-ridden by a kernel parameter. | |
e0d19036 NB |
308 | |
309 | .SS RECOVERY | |
310 | ||
addc80c4 | 311 | If the md driver detects a write error on a device in a RAID1, RAID4, |
599e5a36 NB |
312 | RAID5, RAID6, or RAID10 array, it immediately disables that device |
313 | (marking it as faulty) and continues operation on the remaining | |
314 | devices. If there is a spare drive, the driver will start recreating | |
315 | on one of the spare drives the data what was on that failed drive, | |
316 | either by copying a working drive in a RAID1 configuration, or by | |
317 | doing calculations with the parity block on RAID4, RAID5 or RAID6, or | |
318 | by finding a copying originals for RAID10. | |
e0d19036 | 319 | |
addc80c4 NB |
320 | In kernels prior to about 2.6.15, a read error would cause the same |
321 | effect as a write error. In later kernels, a read-error will instead | |
322 | cause md to attempt a recovery by overwriting the bad block. i.e. it | |
323 | will find the correct data from elsewhere, write it over the block | |
324 | that failed, and then try to read it back again. If either the write | |
325 | or the re-read fail, md will treat the error the same way that a write | |
326 | error is treated and will fail the whole device. | |
327 | ||
2d465520 | 328 | While this recovery process is happening, the md driver will monitor |
e0d19036 NB |
329 | accesses to the array and will slow down the rate of recovery if other |
330 | activity is happening, so that normal access to the array will not be | |
331 | unduly affected. When no other activity is happening, the recovery | |
332 | process proceeds at full speed. The actual speed targets for the two | |
333 | different situations can be controlled by the | |
334 | .B speed_limit_min | |
335 | and | |
336 | .B speed_limit_max | |
337 | control files mentioned below. | |
338 | ||
599e5a36 NB |
339 | .SS BITMAP WRITE-INTENT LOGGING |
340 | ||
341 | From Linux 2.6.13, | |
342 | .I md | |
343 | supports a bitmap based write-intent log. If configured, the bitmap | |
344 | is used to record which blocks of the array may be out of sync. | |
345 | Before any write request is honoured, md will make sure that the | |
346 | corresponding bit in the log is set. After a period of time with no | |
347 | writes to an area of the array, the corresponding bit will be cleared. | |
348 | ||
349 | This bitmap is used for two optimisations. | |
350 | ||
351 | Firstly, after an unclear shutdown, the resync process will consult | |
352 | the bitmap and only resync those blocks that correspond to bits in the | |
353 | bitmap that are set. This can dramatically increase resync time. | |
354 | ||
355 | Secondly, when a drive fails and is removed from the array, md stops | |
356 | clearing bits in the intent log. If that same drive is re-added to | |
357 | the array, md will notice and will only recover the sections of the | |
358 | drive that are covered by bits in the intent log that are set. This | |
359 | can allow a device to be temporarily removed and reinserted without | |
360 | causing an enormous recovery cost. | |
361 | ||
362 | The intent log can be stored in a file on a separate device, or it can | |
363 | be stored near the superblocks of an array which has superblocks. | |
364 | ||
addc80c4 NB |
365 | It is possible to add an intent log or an active array, or remove an |
366 | intent log if one is present. | |
599e5a36 NB |
367 | |
368 | In 2.6.13, intent bitmaps are only supported with RAID1. Other levels | |
addc80c4 | 369 | with redundancy are supported from 2.6.15. |
599e5a36 NB |
370 | |
371 | .SS WRITE-BEHIND | |
372 | ||
373 | From Linux 2.6.14, | |
374 | .I md | |
addc80c4 | 375 | supports WRITE-BEHIND on RAID1 arrays. |
599e5a36 NB |
376 | |
377 | This allows certain devices in the array to be flagged as | |
378 | .IR write-mostly . | |
379 | MD will only read from such devices if there is no | |
380 | other option. | |
381 | ||
382 | If a write-intent bitmap is also provided, write requests to | |
383 | write-mostly devices will be treated as write-behind requests and md | |
384 | will not wait for writes to those requests to complete before | |
385 | reporting the write as complete to the filesystem. | |
386 | ||
387 | This allows for a RAID1 with WRITE-BEHIND to be used to mirror data | |
388 | over a slow link to a remove computer (providing the link isn't too | |
389 | slow). The extra latency of the remote link will not slow down normal | |
390 | operations, but the remote system will still have a reasonably | |
391 | up-to-date copy of all data. | |
392 | ||
addc80c4 NB |
393 | .SS RESTRIPING |
394 | ||
395 | .IR Restriping , | |
396 | also known as | |
397 | .IR Reshaping , | |
398 | is the processes of re-arranging the data stored in each stripe into a | |
399 | new layout. This might involve changing the number of devices in the | |
400 | array (so the stripes are wider) changing the chunk size (so stripes | |
401 | are deeper or shallower), or changing the arrangement of data and | |
402 | parity, possibly changing the raid level (e.g. 1 to 5 or 5 to 6). | |
403 | ||
404 | As of Linux 2.6.17, md can reshape a raid5 array to have more | |
405 | devices. Other possibilities may follow in future kernels. | |
406 | ||
407 | During any stripe process there is a 'critical section' during which | |
408 | live data is being over-written on disk. For the operation of | |
409 | increasing the number of drives in a raid5, this critical section | |
410 | covers the first few stripes (the number being the product of the old | |
411 | and new number of devices). After this critical section is passed, | |
412 | data is only written to areas of the array which no longer hold live | |
413 | data - the live data has already been located away. | |
414 | ||
415 | md is not able to ensure data preservation if there is a crash | |
416 | (e.g. power failure) during the critical section. If md is asked to | |
417 | start an array which failed during a critical section of restriping, | |
418 | it will fail to start the array. | |
419 | ||
420 | To deal with this possibility, a user-space program must | |
421 | .IP \(bu 4 | |
422 | Disable writes to that section of the array (using the | |
423 | .B sysfs | |
424 | interface), | |
425 | .IP \(bu 4 | |
426 | Take a copy of the data somewhere (i.e. make a backup) | |
427 | .IP \(bu 4 | |
428 | Allow the process to continue and invalidate the backup and restore | |
429 | write access once the critical section is passed, and | |
430 | .IP \(bu 4 | |
431 | Provide for restoring the critical data before restarting the array | |
432 | after a system crash. | |
433 | .PP | |
434 | ||
435 | .B mdadm | |
436 | version 2.4 and later will do this for growing a RAID5 array. | |
437 | ||
438 | For operations that do not change the size of the array, like simply | |
439 | increasing chunk size, or converting RAID5 to RAID6 with one extra | |
440 | device, the entire process is the critical section. In this case the | |
441 | restripe will need to progress in stages as a section is suspended, | |
442 | backed up, | |
443 | restriped, and released. This is not yet implemented. | |
444 | ||
445 | .SS SYSFS INTERFACE | |
446 | All block devices appear as a directory in | |
447 | .I sysfs | |
448 | (usually mounted at | |
449 | .BR /sys ). | |
450 | For MD devices, this directory will contain a subdirectory called | |
451 | .B md | |
452 | which contains various files for providing access to information about | |
453 | the array. | |
454 | ||
455 | This interface is documented more fully in the file | |
456 | .B Documentation/md.txt | |
457 | which is distributed with the kernel sources. That file should be | |
458 | consulted for full documentation. The following are just a selection | |
459 | of attribute files that are available. | |
460 | ||
461 | .TP | |
462 | .B md/sync_speed_min | |
463 | This value, if set, overrides the system-wide setting in | |
464 | .B /proc/sys/dev/raid/speed_limit_min | |
465 | for this array only. | |
466 | Writing the value | |
467 | .B system | |
468 | to this file cause the system-wide setting to have effect. | |
469 | ||
470 | .TP | |
471 | .B md/sync_speed_max | |
472 | This is the partner of | |
473 | .B md/sync_speed_min | |
474 | and overrides | |
475 | .B /proc/sys/dev/raid/spool_limit_max | |
476 | described below. | |
477 | ||
478 | .TP | |
479 | .B md/sync_action | |
480 | This can be used to monitor and control the resync/recovery process of | |
481 | MD. | |
482 | In particular, writing "check" here will cause the array to read all | |
483 | data block and check that they are consistent (e.g. parity is correct, | |
484 | or all mirror replicas are the same). Any discrepancies found are | |
485 | .B NOT | |
486 | corrected. | |
487 | ||
488 | A count of problems found will be stored in | |
489 | .BR md/mismatch_count . | |
490 | ||
491 | Alternately, "repair" can be written which will cause the same check | |
492 | to be performed, but any errors will be corrected. | |
493 | ||
494 | Finally, "idle" can be written to stop the check/repair process. | |
495 | ||
496 | .TP | |
497 | .B md/stripe_cache_size | |
498 | This is only available on RAID5 and RAID6. It records the size (in | |
499 | pages per device) of the stripe cache which is used for synchronising | |
500 | all read and write operations to the array. The default is 128. | |
501 | Increasing this number can increase performance in some situations, at | |
502 | some cost in system memory. | |
503 | ||
504 | ||
5787fa49 NB |
505 | .SS KERNEL PARAMETERS |
506 | ||
addc80c4 | 507 | The md driver recognised several different kernel parameters. |
5787fa49 NB |
508 | .TP |
509 | .B raid=noautodetect | |
510 | This will disable the normal detection of md arrays that happens at | |
511 | boot time. If a drive is partitioned with MS-DOS style partitions, | |
512 | then if any of the 4 main partitions has a partition type of 0xFD, | |
513 | then that partition will normally be inspected to see if it is part of | |
514 | an MD array, and if any full arrays are found, they are started. This | |
addc80c4 | 515 | kernel parameter disables this behaviour. |
5787fa49 | 516 | |
a9d69660 NB |
517 | .TP |
518 | .B raid=partitionable | |
519 | .TP | |
520 | .B raid=part | |
521 | These are available in 2.6 and later kernels only. They indicate that | |
522 | autodetected MD arrays should be created as partitionable arrays, with | |
523 | a different major device number to the original non-partitionable md | |
524 | arrays. The device number is listed as | |
525 | .I mdp | |
526 | in | |
527 | .IR /proc/devices . | |
528 | ||
addc80c4 NB |
529 | .TP |
530 | .B md_mod.start_ro=1 | |
531 | This tells md to start all arrays in read-only mode. This is a soft | |
532 | read-only that will automatically switch to read-write on the first | |
533 | write request. However until that write request, nothing is written | |
534 | to any device by md, and in particular, no resync or recovery | |
535 | operation is started. | |
536 | ||
537 | .TP | |
538 | .B md_mod.start_dirty_degraded=1 | |
539 | As mentioned above, md will not normally start a RAID4, RAID5, or | |
540 | RAID6 that is both dirty and degraded as this situation can imply | |
541 | hidden data loss. This can be awkward if the root filesystem is | |
542 | affected. Using the module parameter allows such arrays to be started | |
543 | at boot time. It should be understood that there is a real (though | |
544 | small) risk of data corruption in this situation. | |
a9d69660 | 545 | |
5787fa49 NB |
546 | .TP |
547 | .BI md= n , dev , dev ,... | |
a9d69660 NB |
548 | .TP |
549 | .BI md=d n , dev , dev ,... | |
5787fa49 NB |
550 | This tells the md driver to assemble |
551 | .B /dev/md n | |
552 | from the listed devices. It is only necessary to start the device | |
553 | holding the root filesystem this way. Other arrays are best started | |
554 | once the system is booted. | |
555 | ||
a9d69660 NB |
556 | In 2.6 kernels, the |
557 | .B d | |
558 | immediately after the | |
559 | .B = | |
560 | indicates that a partitionable device (e.g. | |
561 | .BR /dev/md/d0 ) | |
562 | should be created rather than the original non-partitionable device. | |
563 | ||
5787fa49 NB |
564 | .TP |
565 | .BI md= n , l , c , i , dev... | |
566 | This tells the md driver to assemble a legacy RAID0 or LINEAR array | |
567 | without a superblock. | |
568 | .I n | |
569 | gives the md device number, | |
570 | .I l | |
571 | gives the level, 0 for RAID0 or -1 for LINEAR, | |
572 | .I c | |
573 | gives the chunk size as a base-2 logarithm offset by twelve, so 0 | |
574 | means 4K, 1 means 8K. | |
575 | .I i | |
576 | is ignored (legacy support). | |
e0d19036 | 577 | |
56eb10c0 NB |
578 | .SH FILES |
579 | .TP | |
580 | .B /proc/mdstat | |
581 | Contains information about the status of currently running array. | |
582 | .TP | |
583 | .B /proc/sys/dev/raid/speed_limit_min | |
584 | A readable and writable file that reflects the current goal rebuild | |
585 | speed for times when non-rebuild activity is current on an array. | |
586 | The speed is in Kibibytes per second, and is a per-device rate, not a | |
587 | per-array rate (which means that an array with more disc will shuffle | |
588 | more data for a given speed). The default is 100. | |
589 | ||
590 | .TP | |
591 | .B /proc/sys/dev/raid/speed_limit_max | |
592 | A readable and writable file that reflects the current goal rebuild | |
593 | speed for times when no non-rebuild activity is current on an array. | |
594 | The default is 100,000. | |
595 | ||
596 | .SH SEE ALSO | |
597 | .BR mdadm (8), | |
598 | .BR mkraid (8). |