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Some fixes to the mapfile code.
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1.\" Copyright Neil Brown and others.
2.\" This program is free software; you can redistribute it and/or modify
3.\" it under the terms of the GNU General Public License as published by
4.\" the Free Software Foundation; either version 2 of the License, or
5.\" (at your option) any later version.
6.\" See file COPYING in distribution for details.
7.TH MD 4
8.SH NAME
9md \- Multiple Device driver aka Linux Software RAID
10.SH SYNOPSIS
11.BI /dev/md n
12.br
13.BI /dev/md/ n
14.br
15.BR /dev/md/ name
16.SH DESCRIPTION
17The
18.B md
19driver provides virtual devices that are created from one or more
20independent underlying devices. This array of devices often contains
21redundancy and the devices are often disk drives, hence the acronym RAID
22which stands for a Redundant Array of Independent Disks.
23.PP
24.B md
25supports RAID levels
261 (mirroring),
274 (striped array with parity device),
285 (striped array with distributed parity information),
296 (striped array with distributed dual redundancy information), and
3010 (striped and mirrored).
31If some number of underlying devices fails while using one of these
32levels, the array will continue to function; this number is one for
33RAID levels 4 and 5, two for RAID level 6, and all but one (N-1) for
34RAID level 1, and dependent on configuration for level 10.
35.PP
36.B md
37also supports a number of pseudo RAID (non-redundant) configurations
38including RAID0 (striped array), LINEAR (catenated array),
39MULTIPATH (a set of different interfaces to the same device),
40and FAULTY (a layer over a single device into which errors can be injected).
41
42.SS MD METADATA
43Each device in an array may have some
44.I metadata
45stored in the device. This metadata is sometimes called a
46.BR superblock .
47The metadata records information about the structure and state of the array.
48This allows the array to be reliably re-assembled after a shutdown.
49
50From Linux kernel version 2.6.10,
51.B md
52provides support for two different formats of metadata, and
53other formats can be added. Prior to this release, only one format is
54supported.
55
56The common format \(em known as version 0.90 \(em has
57a superblock that is 4K long and is written into a 64K aligned block that
58starts at least 64K and less than 128K from the end of the device
59(i.e. to get the address of the superblock round the size of the
60device down to a multiple of 64K and then subtract 64K).
61The available size of each device is the amount of space before the
62super block, so between 64K and 128K is lost when a device in
63incorporated into an MD array.
64This superblock stores multi-byte fields in a processor-dependent
65manner, so arrays cannot easily be moved between computers with
66different processors.
67
68The new format \(em known as version 1 \(em has a superblock that is
69normally 1K long, but can be longer. It is normally stored between 8K
70and 12K from the end of the device, on a 4K boundary, though
71variations can be stored at the start of the device (version 1.1) or 4K from
72the start of the device (version 1.2).
73This metadata format stores multibyte data in a
74processor-independent format and supports up to hundreds of
75component devices (version 0.90 only supports 28).
76
77The metadata contains, among other things:
78.TP
79LEVEL
80The manner in which the devices are arranged into the array
81(linear, raid0, raid1, raid4, raid5, raid10, multipath).
82.TP
83UUID
84a 128 bit Universally Unique Identifier that identifies the array that
85contains this device.
86
87.PP
88When a version 0.90 array is being reshaped (e.g. adding extra devices
89to a RAID5), the version number is temporarily set to 0.91. This
90ensures that if the reshape process is stopped in the middle (e.g. by
91a system crash) and the machine boots into an older kernel that does
92not support reshaping, then the array will not be assembled (which
93would cause data corruption) but will be left untouched until a kernel
94that can complete the reshape processes is used.
95
96.SS ARRAYS WITHOUT METADATA
97While it is usually best to create arrays with superblocks so that
98they can be assembled reliably, there are some circumstances when an
99array without superblocks is preferred. These include:
100.TP
101LEGACY ARRAYS
102Early versions of the
103.B md
104driver only supported Linear and Raid0 configurations and did not use
105a superblock (which is less critical with these configurations).
106While such arrays should be rebuilt with superblocks if possible,
107.B md
108continues to support them.
109.TP
110FAULTY
111Being a largely transparent layer over a different device, the FAULTY
112personality doesn't gain anything from having a superblock.
113.TP
114MULTIPATH
115It is often possible to detect devices which are different paths to
116the same storage directly rather than having a distinctive superblock
117written to the device and searched for on all paths. In this case,
118a MULTIPATH array with no superblock makes sense.
119.TP
120RAID1
121In some configurations it might be desired to create a raid1
122configuration that does not use a superblock, and to maintain the state of
123the array elsewhere. While not encouraged for general us, it does
124have special-purpose uses and is supported.
125
126.SS ARRAYS WITH EXTERNAL METADATA
127
128From release 2.6.28, the
129.I md
130driver supports arrays with externally managed metadata. That is,
131the metadata is not managed by the kernel by rather by a user-space
132program which is external to the kernel. This allows support for a
133variety of metadata formats without cluttering the kernel with lots of
134details.
135.PP
136.I md
137is able to communicate with the user-space program through various
138sysfs attributes so that it can make appropriate changes to the
139metadata \- for example to make a device as faulty. When necessary,
140.I md
141will wait for the program to acknowledge the event by writing to a
142sysfs attribute.
143The manual page for
144.IR mdmon (8)
145contains more detail about this interaction.
146
147.SS CONTAINERS
148Many metadata formats use a single block of metadata to describe a
149number of different arrays which all use the same set of devices.
150In this case it is helpful for the kernel to know about the full set
151of devices as a whole. This set is known to md as a
152.IR container .
153A container is an
154.I md
155array with externally managed metadata and with device offset and size
156so that it just covers the metadata part of the devices. The
157remainder of each device is available to be incorporated into various
158arrays.
159
160.SS LINEAR
161
162A linear array simply catenates the available space on each
163drive to form one large virtual drive.
164
165One advantage of this arrangement over the more common RAID0
166arrangement is that the array may be reconfigured at a later time with
167an extra drive, so the array is made bigger without disturbing the
168data that is on the array. This can even be done on a live
169array.
170
171If a chunksize is given with a LINEAR array, the usable space on each
172device is rounded down to a multiple of this chunksize.
173
174.SS RAID0
175
176A RAID0 array (which has zero redundancy) is also known as a
177striped array.
178A RAID0 array is configured at creation with a
179.B "Chunk Size"
180which must be a power of two (prior to Linux 2.6.31), and at least 4
181kibibytes.
182
183The RAID0 driver assigns the first chunk of the array to the first
184device, the second chunk to the second device, and so on until all
185drives have been assigned one chunk. This collection of chunks forms a
186.BR stripe .
187Further chunks are gathered into stripes in the same way, and are
188assigned to the remaining space in the drives.
189
190If devices in the array are not all the same size, then once the
191smallest device has been exhausted, the RAID0 driver starts
192collecting chunks into smaller stripes that only span the drives which
193still have remaining space.
194
195
196.SS RAID1
197
198A RAID1 array is also known as a mirrored set (though mirrors tend to
199provide reflected images, which RAID1 does not) or a plex.
200
201Once initialised, each device in a RAID1 array contains exactly the
202same data. Changes are written to all devices in parallel. Data is
203read from any one device. The driver attempts to distribute read
204requests across all devices to maximise performance.
205
206All devices in a RAID1 array should be the same size. If they are
207not, then only the amount of space available on the smallest device is
208used (any extra space on other devices is wasted).
209
210Note that the read balancing done by the driver does not make the RAID1
211performance profile be the same as for RAID0; a single stream of
212sequential input will not be accelerated (e.g. a single dd), but
213multiple sequential streams or a random workload will use more than one
214spindle. In theory, having an N-disk RAID1 will allow N sequential
215threads to read from all disks.
216
217Individual devices in a RAID1 can be marked as "write-mostly".
218This drives are excluded from the normal read balancing and will only
219be read from when there is no other option. This can be useful for
220devices connected over a slow link.
221
222.SS RAID4
223
224A RAID4 array is like a RAID0 array with an extra device for storing
225parity. This device is the last of the active devices in the
226array. Unlike RAID0, RAID4 also requires that all stripes span all
227drives, so extra space on devices that are larger than the smallest is
228wasted.
229
230When any block in a RAID4 array is modified, the parity block for that
231stripe (i.e. the block in the parity device at the same device offset
232as the stripe) is also modified so that the parity block always
233contains the "parity" for the whole stripe. I.e. its content is
234equivalent to the result of performing an exclusive-or operation
235between all the data blocks in the stripe.
236
237This allows the array to continue to function if one device fails.
238The data that was on that device can be calculated as needed from the
239parity block and the other data blocks.
240
241.SS RAID5
242
243RAID5 is very similar to RAID4. The difference is that the parity
244blocks for each stripe, instead of being on a single device, are
245distributed across all devices. This allows more parallelism when
246writing, as two different block updates will quite possibly affect
247parity blocks on different devices so there is less contention.
248
249This also allows more parallelism when reading, as read requests are
250distributed over all the devices in the array instead of all but one.
251
252.SS RAID6
253
254RAID6 is similar to RAID5, but can handle the loss of any \fItwo\fP
255devices without data loss. Accordingly, it requires N+2 drives to
256store N drives worth of data.
257
258The performance for RAID6 is slightly lower but comparable to RAID5 in
259normal mode and single disk failure mode. It is very slow in dual
260disk failure mode, however.
261
262.SS RAID10
263
264RAID10 provides a combination of RAID1 and RAID0, and is sometimes known
265as RAID1+0. Every datablock is duplicated some number of times, and
266the resulting collection of datablocks are distributed over multiple
267drives.
268
269When configuring a RAID10 array, it is necessary to specify the number
270of replicas of each data block that are required (this will normally
271be 2) and whether the replicas should be 'near', 'offset' or 'far'.
272(Note that the 'offset' layout is only available from 2.6.18).
273
274When 'near' replicas are chosen, the multiple copies of a given chunk
275are laid out consecutively across the stripes of the array, so the two
276copies of a datablock will likely be at the same offset on two
277adjacent devices.
278
279When 'far' replicas are chosen, the multiple copies of a given chunk
280are laid out quite distant from each other. The first copy of all
281data blocks will be striped across the early part of all drives in
282RAID0 fashion, and then the next copy of all blocks will be striped
283across a later section of all drives, always ensuring that all copies
284of any given block are on different drives.
285
286The 'far' arrangement can give sequential read performance equal to
287that of a RAID0 array, but at the cost of reduced write performance.
288
289When 'offset' replicas are chosen, the multiple copies of a given
290chunk are laid out on consecutive drives and at consecutive offsets.
291Effectively each stripe is duplicated and the copies are offset by one
292device. This should give similar read characteristics to 'far' if a
293suitably large chunk size is used, but without as much seeking for
294writes.
295
296It should be noted that the number of devices in a RAID10 array need
297not be a multiple of the number of replica of each data block; however,
298there must be at least as many devices as replicas.
299
300If, for example, an array is created with 5 devices and 2 replicas,
301then space equivalent to 2.5 of the devices will be available, and
302every block will be stored on two different devices.
303
304Finally, it is possible to have an array with both 'near' and 'far'
305copies. If an array is configured with 2 near copies and 2 far
306copies, then there will be a total of 4 copies of each block, each on
307a different drive. This is an artifact of the implementation and is
308unlikely to be of real value.
309
310.SS MULTIPATH
311
312MULTIPATH is not really a RAID at all as there is only one real device
313in a MULTIPATH md array. However there are multiple access points
314(paths) to this device, and one of these paths might fail, so there
315are some similarities.
316
317A MULTIPATH array is composed of a number of logically different
318devices, often fibre channel interfaces, that all refer the the same
319real device. If one of these interfaces fails (e.g. due to cable
320problems), the multipath driver will attempt to redirect requests to
321another interface.
322
323The MULTIPATH drive is not receiving any ongoing development and
324should be considered a legacy driver. The device-mapper based
325multipath drivers should be preferred for new installations.
326
327.SS FAULTY
328The FAULTY md module is provided for testing purposes. A faulty array
329has exactly one component device and is normally assembled without a
330superblock, so the md array created provides direct access to all of
331the data in the component device.
332
333The FAULTY module may be requested to simulate faults to allow testing
334of other md levels or of filesystems. Faults can be chosen to trigger
335on read requests or write requests, and can be transient (a subsequent
336read/write at the address will probably succeed) or persistent
337(subsequent read/write of the same address will fail). Further, read
338faults can be "fixable" meaning that they persist until a write
339request at the same address.
340
341Fault types can be requested with a period. In this case, the fault
342will recur repeatedly after the given number of requests of the
343relevant type. For example if persistent read faults have a period of
344100, then every 100th read request would generate a fault, and the
345faulty sector would be recorded so that subsequent reads on that
346sector would also fail.
347
348There is a limit to the number of faulty sectors that are remembered.
349Faults generated after this limit is exhausted are treated as
350transient.
351
352The list of faulty sectors can be flushed, and the active list of
353failure modes can be cleared.
354
355.SS UNCLEAN SHUTDOWN
356
357When changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array
358there is a possibility of inconsistency for short periods of time as
359each update requires at least two block to be written to different
360devices, and these writes probably won't happen at exactly the same
361time. Thus if a system with one of these arrays is shutdown in the
362middle of a write operation (e.g. due to power failure), the array may
363not be consistent.
364
365To handle this situation, the md driver marks an array as "dirty"
366before writing any data to it, and marks it as "clean" when the array
367is being disabled, e.g. at shutdown. If the md driver finds an array
368to be dirty at startup, it proceeds to correct any possibly
369inconsistency. For RAID1, this involves copying the contents of the
370first drive onto all other drives. For RAID4, RAID5 and RAID6 this
371involves recalculating the parity for each stripe and making sure that
372the parity block has the correct data. For RAID10 it involves copying
373one of the replicas of each block onto all the others. This process,
374known as "resynchronising" or "resync" is performed in the background.
375The array can still be used, though possibly with reduced performance.
376
377If a RAID4, RAID5 or RAID6 array is degraded (missing at least one
378drive, two for RAID6) when it is restarted after an unclean shutdown, it cannot
379recalculate parity, and so it is possible that data might be
380undetectably corrupted. The 2.4 md driver
381.B does not
382alert the operator to this condition. The 2.6 md driver will fail to
383start an array in this condition without manual intervention, though
384this behaviour can be overridden by a kernel parameter.
385
386.SS RECOVERY
387
388If the md driver detects a write error on a device in a RAID1, RAID4,
389RAID5, RAID6, or RAID10 array, it immediately disables that device
390(marking it as faulty) and continues operation on the remaining
391devices. If there are spare drives, the driver will start recreating
392on one of the spare drives the data which was on that failed drive,
393either by copying a working drive in a RAID1 configuration, or by
394doing calculations with the parity block on RAID4, RAID5 or RAID6, or
395by finding and copying originals for RAID10.
396
397In kernels prior to about 2.6.15, a read error would cause the same
398effect as a write error. In later kernels, a read-error will instead
399cause md to attempt a recovery by overwriting the bad block. i.e. it
400will find the correct data from elsewhere, write it over the block
401that failed, and then try to read it back again. If either the write
402or the re-read fail, md will treat the error the same way that a write
403error is treated, and will fail the whole device.
404
405While this recovery process is happening, the md driver will monitor
406accesses to the array and will slow down the rate of recovery if other
407activity is happening, so that normal access to the array will not be
408unduly affected. When no other activity is happening, the recovery
409process proceeds at full speed. The actual speed targets for the two
410different situations can be controlled by the
411.B speed_limit_min
412and
413.B speed_limit_max
414control files mentioned below.
415
416.SS SCRUBBING AND MISMATCHES
417
418As storage devices can develop bad blocks at any time it is valuable
419to regularly read all blocks on all devices in an array so as to catch
420such bad blocks early. This process is called
421.IR scrubbing .
422
423md arrays can be scrubbed by writing either
424.I check
425or
426.I repair
427to the file
428.I md/sync_action
429in the
430.I sysfs
431directory for the device.
432
433Requesting a scrub will cause
434.I md
435to read every block on every device in the array, and check that the
436data is consistent. For RAID1 and RAID10, this means checking that the copies
437are identical. For RAID4, RAID5, RAID6 this means checking that the
438parity block is (or blocks are) correct.
439
440If a read error is detected during this process, the normal read-error
441handling causes correct data to be found from other devices and to be
442written back to the faulty device. In many case this will
443effectively
444.I fix
445the bad block.
446
447If all blocks read successfully but are found to not be consistent,
448then this is regarded as a
449.IR mismatch .
450
451If
452.I check
453was used, then no action is taken to handle the mismatch, it is simply
454recorded.
455If
456.I repair
457was used, then a mismatch will be repaired in the same way that
458.I resync
459repairs arrays. For RAID5/RAID6 new parity blocks are written. For RAID1/RAID10,
460all but one block are overwritten with the content of that one block.
461
462A count of mismatches is recorded in the
463.I sysfs
464file
465.IR md/mismatch_cnt .
466This is set to zero when a
467scrub starts and is incremented whenever a sector is
468found that is a mismatch.
469.I md
470normally works in units much larger than a single sector and when it
471finds a mismatch, it does not determin exactly how many actual sectors were
472affected but simply adds the number of sectors in the IO unit that was
473used. So a value of 128 could simply mean that a single 64KB check
474found an error (128 x 512bytes = 64KB).
475
476If an array is created by
477.I mdadm
478with
479.I \-\-assume\-clean
480then a subsequent check could be expected to find some mismatches.
481
482On a truly clean RAID5 or RAID6 array, any mismatches should indicate
483a hardware problem at some level - software issues should never cause
484such a mismatch.
485
486However on RAID1 and RAID10 it is possible for software issues to
487cause a mismatch to be reported. This does not necessarily mean that
488the data on the array is corrupted. It could simply be that the
489system does not care what is stored on that part of the array - it is
490unused space.
491
492The most likely cause for an unexpected mismatch on RAID1 or RAID10
493occurs if a swap partition or swap file is stored on the array.
494
495When the swap subsystem wants to write a page of memory out, it flags
496the page as 'clean' in the memory manager and requests the swap device
497to write it out. It is quite possible that the memory will be
498changed while the write-out is happening. In that case the 'clean'
499flag will be found to be clear when the write completes and so the
500swap subsystem will simply forget that the swapout had been attempted,
501and will possibly choose a different page to write out.
502
503If the swap device was on RAID1 (or RAID10), then the data is sent
504from memory to a device twice (or more depending on the number of
505devices in the array). Thus it is possible that the memory gets changed
506between the times it is sent, so different data can be written to
507the different devices in the array. This will be detected by
508.I check
509as a mismatch. However it does not reflect any corruption as the
510block where this mismatch occurs is being treated by the swap system as
511being empty, and the data will never be read from that block.
512
513It is conceivable for a similar situation to occur on non-swap files,
514though it is less likely.
515
516Thus the
517.I mismatch_cnt
518value can not be interpreted very reliably on RAID1 or RAID10,
519especially when the device is used for swap.
520
521
522.SS BITMAP WRITE-INTENT LOGGING
523
524From Linux 2.6.13,
525.I md
526supports a bitmap based write-intent log. If configured, the bitmap
527is used to record which blocks of the array may be out of sync.
528Before any write request is honoured, md will make sure that the
529corresponding bit in the log is set. After a period of time with no
530writes to an area of the array, the corresponding bit will be cleared.
531
532This bitmap is used for two optimisations.
533
534Firstly, after an unclean shutdown, the resync process will consult
535the bitmap and only resync those blocks that correspond to bits in the
536bitmap that are set. This can dramatically reduce resync time.
537
538Secondly, when a drive fails and is removed from the array, md stops
539clearing bits in the intent log. If that same drive is re-added to
540the array, md will notice and will only recover the sections of the
541drive that are covered by bits in the intent log that are set. This
542can allow a device to be temporarily removed and reinserted without
543causing an enormous recovery cost.
544
545The intent log can be stored in a file on a separate device, or it can
546be stored near the superblocks of an array which has superblocks.
547
548It is possible to add an intent log to an active array, or remove an
549intent log if one is present.
550
551In 2.6.13, intent bitmaps are only supported with RAID1. Other levels
552with redundancy are supported from 2.6.15.
553
554.SS WRITE-BEHIND
555
556From Linux 2.6.14,
557.I md
558supports WRITE-BEHIND on RAID1 arrays.
559
560This allows certain devices in the array to be flagged as
561.IR write-mostly .
562MD will only read from such devices if there is no
563other option.
564
565If a write-intent bitmap is also provided, write requests to
566write-mostly devices will be treated as write-behind requests and md
567will not wait for writes to those requests to complete before
568reporting the write as complete to the filesystem.
569
570This allows for a RAID1 with WRITE-BEHIND to be used to mirror data
571over a slow link to a remote computer (providing the link isn't too
572slow). The extra latency of the remote link will not slow down normal
573operations, but the remote system will still have a reasonably
574up-to-date copy of all data.
575
576.SS RESTRIPING
577
578.IR Restriping ,
579also known as
580.IR Reshaping ,
581is the processes of re-arranging the data stored in each stripe into a
582new layout. This might involve changing the number of devices in the
583array (so the stripes are wider), changing the chunk size (so stripes
584are deeper or shallower), or changing the arrangement of data and
585parity (possibly changing the raid level, e.g. 1 to 5 or 5 to 6).
586
587As of Linux 2.6.17, md can reshape a raid5 array to have more
588devices. Other possibilities may follow in future kernels.
589
590During any stripe process there is a 'critical section' during which
591live data is being overwritten on disk. For the operation of
592increasing the number of drives in a raid5, this critical section
593covers the first few stripes (the number being the product of the old
594and new number of devices). After this critical section is passed,
595data is only written to areas of the array which no longer hold live
596data \(em the live data has already been located away.
597
598md is not able to ensure data preservation if there is a crash
599(e.g. power failure) during the critical section. If md is asked to
600start an array which failed during a critical section of restriping,
601it will fail to start the array.
602
603To deal with this possibility, a user-space program must
604.IP \(bu 4
605Disable writes to that section of the array (using the
606.B sysfs
607interface),
608.IP \(bu 4
609take a copy of the data somewhere (i.e. make a backup),
610.IP \(bu 4
611allow the process to continue and invalidate the backup and restore
612write access once the critical section is passed, and
613.IP \(bu 4
614provide for restoring the critical data before restarting the array
615after a system crash.
616.PP
617
618.B mdadm
619versions from 2.4 do this for growing a RAID5 array.
620
621For operations that do not change the size of the array, like simply
622increasing chunk size, or converting RAID5 to RAID6 with one extra
623device, the entire process is the critical section. In this case, the
624restripe will need to progress in stages, as a section is suspended,
625backed up,
626restriped, and released; this is not yet implemented.
627
628.SS SYSFS INTERFACE
629Each block device appears as a directory in
630.I sysfs
631(which is usually mounted at
632.BR /sys ).
633For MD devices, this directory will contain a subdirectory called
634.B md
635which contains various files for providing access to information about
636the array.
637
638This interface is documented more fully in the file
639.B Documentation/md.txt
640which is distributed with the kernel sources. That file should be
641consulted for full documentation. The following are just a selection
642of attribute files that are available.
643
644.TP
645.B md/sync_speed_min
646This value, if set, overrides the system-wide setting in
647.B /proc/sys/dev/raid/speed_limit_min
648for this array only.
649Writing the value
650.B "system"
651to this file will cause the system-wide setting to have effect.
652
653.TP
654.B md/sync_speed_max
655This is the partner of
656.B md/sync_speed_min
657and overrides
658.B /proc/sys/dev/raid/spool_limit_max
659described below.
660
661.TP
662.B md/sync_action
663This can be used to monitor and control the resync/recovery process of
664MD.
665In particular, writing "check" here will cause the array to read all
666data block and check that they are consistent (e.g. parity is correct,
667or all mirror replicas are the same). Any discrepancies found are
668.B NOT
669corrected.
670
671A count of problems found will be stored in
672.BR md/mismatch_count .
673
674Alternately, "repair" can be written which will cause the same check
675to be performed, but any errors will be corrected.
676
677Finally, "idle" can be written to stop the check/repair process.
678
679.TP
680.B md/stripe_cache_size
681This is only available on RAID5 and RAID6. It records the size (in
682pages per device) of the stripe cache which is used for synchronising
683all write operations to the array and all read operations if the array
684is degraded. The default is 256. Valid values are 17 to 32768.
685Increasing this number can increase performance in some situations, at
686some cost in system memory. Note, setting this value too high can
687result in an "out of memory" condition for the system.
688
689memory_consumed = system_page_size * nr_disks * stripe_cache_size
690
691.TP
692.B md/preread_bypass_threshold
693This is only available on RAID5 and RAID6. This variable sets the
694number of times MD will service a full-stripe-write before servicing a
695stripe that requires some "prereading". For fairness this defaults to
6961. Valid values are 0 to stripe_cache_size. Setting this to 0
697maximizes sequential-write throughput at the cost of fairness to threads
698doing small or random writes.
699
700.SS KERNEL PARAMETERS
701
702The md driver recognised several different kernel parameters.
703.TP
704.B raid=noautodetect
705This will disable the normal detection of md arrays that happens at
706boot time. If a drive is partitioned with MS-DOS style partitions,
707then if any of the 4 main partitions has a partition type of 0xFD,
708then that partition will normally be inspected to see if it is part of
709an MD array, and if any full arrays are found, they are started. This
710kernel parameter disables this behaviour.
711
712.TP
713.B raid=partitionable
714.TP
715.B raid=part
716These are available in 2.6 and later kernels only. They indicate that
717autodetected MD arrays should be created as partitionable arrays, with
718a different major device number to the original non-partitionable md
719arrays. The device number is listed as
720.I mdp
721in
722.IR /proc/devices .
723
724.TP
725.B md_mod.start_ro=1
726.TP
727.B /sys/module/md_mod/parameters/start_ro
728This tells md to start all arrays in read-only mode. This is a soft
729read-only that will automatically switch to read-write on the first
730write request. However until that write request, nothing is written
731to any device by md, and in particular, no resync or recovery
732operation is started.
733
734.TP
735.B md_mod.start_dirty_degraded=1
736.TP
737.B /sys/module/md_mod/parameters/start_dirty_degraded
738As mentioned above, md will not normally start a RAID4, RAID5, or
739RAID6 that is both dirty and degraded as this situation can imply
740hidden data loss. This can be awkward if the root filesystem is
741affected. Using this module parameter allows such arrays to be started
742at boot time. It should be understood that there is a real (though
743small) risk of data corruption in this situation.
744
745.TP
746.BI md= n , dev , dev ,...
747.TP
748.BI md=d n , dev , dev ,...
749This tells the md driver to assemble
750.B /dev/md n
751from the listed devices. It is only necessary to start the device
752holding the root filesystem this way. Other arrays are best started
753once the system is booted.
754
755In 2.6 kernels, the
756.B d
757immediately after the
758.B =
759indicates that a partitionable device (e.g.
760.BR /dev/md/d0 )
761should be created rather than the original non-partitionable device.
762
763.TP
764.BI md= n , l , c , i , dev...
765This tells the md driver to assemble a legacy RAID0 or LINEAR array
766without a superblock.
767.I n
768gives the md device number,
769.I l
770gives the level, 0 for RAID0 or -1 for LINEAR,
771.I c
772gives the chunk size as a base-2 logarithm offset by twelve, so 0
773means 4K, 1 means 8K.
774.I i
775is ignored (legacy support).
776
777.SH FILES
778.TP
779.B /proc/mdstat
780Contains information about the status of currently running array.
781.TP
782.B /proc/sys/dev/raid/speed_limit_min
783A readable and writable file that reflects the current "goal" rebuild
784speed for times when non-rebuild activity is current on an array.
785The speed is in Kibibytes per second, and is a per-device rate, not a
786per-array rate (which means that an array with more disks will shuffle
787more data for a given speed). The default is 1000.
788
789.TP
790.B /proc/sys/dev/raid/speed_limit_max
791A readable and writable file that reflects the current "goal" rebuild
792speed for times when no non-rebuild activity is current on an array.
793The default is 200,000.
794
795.SH SEE ALSO
796.BR mdadm (8),
797.BR mkraid (8).