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Based on kernel version 4.9. Page generated on 2016-12-21 14:35 EST.

1	Tools that manage md devices can be found at
2	   http://www.kernel.org/pub/linux/utils/raid/ 
5	Boot time assembly of RAID arrays
6	---------------------------------
8	You can boot with your md device with the following kernel command
9	lines:
11	for old raid arrays without persistent superblocks:
12	  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
14	for raid arrays with persistent superblocks
15	  md=<md device no.>,dev0,dev1,...,devn
16	or, to assemble a partitionable array:
17	  md=d<md device no.>,dev0,dev1,...,devn
19	md device no. = the number of the md device ... 
20	              0 means md0, 
21		      1 md1,
22		      2 md2,
23		      3 md3,
24		      4 md4
26	raid level = -1 linear mode
27	              0 striped mode
28		      other modes are only supported with persistent super blocks
30	chunk size factor = (raid-0 and raid-1 only)
31	              Set  the chunk size as 4k << n.
33	fault level = totally ignored
35	dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
37	A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this:
39	e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
42	Boot time autodetection of RAID arrays
43	--------------------------------------
45	When md is compiled into the kernel (not as module), partitions of
46	type 0xfd are scanned and automatically assembled into RAID arrays.
47	This autodetection may be suppressed with the kernel parameter
48	"raid=noautodetect".  As of kernel 2.6.9, only drives with a type 0
49	superblock can be autodetected and run at boot time.
51	The kernel parameter "raid=partitionable" (or "raid=part") means
52	that all auto-detected arrays are assembled as partitionable.
54	Boot time assembly of degraded/dirty arrays
55	-------------------------------------------
57	If a raid5 or raid6 array is both dirty and degraded, it could have
58	undetectable data corruption.  This is because the fact that it is
59	'dirty' means that the parity cannot be trusted, and the fact that it
60	is degraded means that some datablocks are missing and cannot reliably
61	be reconstructed (due to no parity).
63	For this reason, md will normally refuse to start such an array.  This
64	requires the sysadmin to take action to explicitly start the array
65	despite possible corruption.  This is normally done with
66	   mdadm --assemble --force ....
68	This option is not really available if the array has the root
69	filesystem on it.  In order to support this booting from such an
70	array, md supports a module parameter "start_dirty_degraded" which,
71	when set to 1, bypassed the checks and will allows dirty degraded
72	arrays to be started.
74	So, to boot with a root filesystem of a dirty degraded raid[56], use
76	   md-mod.start_dirty_degraded=1
79	Superblock formats
80	------------------
82	The md driver can support a variety of different superblock formats.
83	Currently, it supports superblock formats "0.90.0" and the "md-1" format
84	introduced in the 2.5 development series.
86	The kernel will autodetect which format superblock is being used.
88	Superblock format '0' is treated differently to others for legacy
89	reasons - it is the original superblock format.
92	General Rules - apply for all superblock formats
93	------------------------------------------------
95	An array is 'created' by writing appropriate superblocks to all
96	devices.
98	It is 'assembled' by associating each of these devices with an
99	particular md virtual device.  Once it is completely assembled, it can
100	be accessed.
102	An array should be created by a user-space tool.  This will write
103	superblocks to all devices.  It will usually mark the array as
104	'unclean', or with some devices missing so that the kernel md driver
105	can create appropriate redundancy (copying in raid1, parity
106	calculation in raid4/5).
108	When an array is assembled, it is first initialized with the
109	SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
110	version number.  The major version number selects which superblock
111	format is to be used.  The minor number might be used to tune handling
112	of the format, such as suggesting where on each device to look for the
113	superblock.
115	Then each device is added using the ADD_NEW_DISK ioctl.  This
116	provides, in particular, a major and minor number identifying the
117	device to add.
119	The array is started with the RUN_ARRAY ioctl.
121	Once started, new devices can be added.  They should have an
122	appropriate superblock written to them, and then be passed in with
125	Devices that have failed or are not yet active can be detached from an
126	array using HOT_REMOVE_DISK.
129	Specific Rules that apply to format-0 super block arrays, and
130	       arrays with no superblock (non-persistent).
131	-------------------------------------------------------------
133	An array can be 'created' by describing the array (level, chunksize
134	etc) in a SET_ARRAY_INFO ioctl.  This must have major_version==0 and
135	raid_disks != 0.
137	Then uninitialized devices can be added with ADD_NEW_DISK.  The
138	structure passed to ADD_NEW_DISK must specify the state of the device
139	and its role in the array.
141	Once started with RUN_ARRAY, uninitialized spares can be added with
146	MD devices in sysfs
147	-------------------
148	md devices appear in sysfs (/sys) as regular block devices,
149	e.g.
150	   /sys/block/md0
152	Each 'md' device will contain a subdirectory called 'md' which
153	contains further md-specific information about the device.
155	All md devices contain:
156	  level
157	     a text file indicating the 'raid level'. e.g. raid0, raid1,
158	     raid5, linear, multipath, faulty.
159	     If no raid level has been set yet (array is still being
160	     assembled), the value will reflect whatever has been written
161	     to it, which may be a name like the above, or may be a number
162	     such as '0', '5', etc.
164	  raid_disks
165	     a text file with a simple number indicating the number of devices
166	     in a fully functional array.  If this is not yet known, the file
167	     will be empty.  If an array is being resized this will contain
168	     the new number of devices.
169	     Some raid levels allow this value to be set while the array is
170	     active.  This will reconfigure the array.   Otherwise it can only
171	     be set while assembling an array.
172	     A change to this attribute will not be permitted if it would
173	     reduce the size of the array.  To reduce the number of drives
174	     in an e.g. raid5, the array size must first be reduced by
175	     setting the 'array_size' attribute.
177	  chunk_size
178	     This is the size in bytes for 'chunks' and is only relevant to
179	     raid levels that involve striping (0,4,5,6,10). The address space
180	     of the array is conceptually divided into chunks and consecutive
181	     chunks are striped onto neighbouring devices.
182	     The size should be at least PAGE_SIZE (4k) and should be a power
183	     of 2.  This can only be set while assembling an array
185	  layout
186	     The "layout" for the array for the particular level.  This is
187	     simply a number that is interpretted differently by different
188	     levels.  It can be written while assembling an array.
190	  array_size
191	     This can be used to artificially constrain the available space in
192	     the array to be less than is actually available on the combined
193	     devices.  Writing a number (in Kilobytes) which is less than
194	     the available size will set the size.  Any reconfiguration of the
195	     array (e.g. adding devices) will not cause the size to change.
196	     Writing the word 'default' will cause the effective size of the
197	     array to be whatever size is actually available based on
198	     'level', 'chunk_size' and 'component_size'.
200	     This can be used to reduce the size of the array before reducing
201	     the number of devices in a raid4/5/6, or to support external
202	     metadata formats which mandate such clipping.
204	  reshape_position
205	     This is either "none" or a sector number within the devices of
206	     the array where "reshape" is up to.  If this is set, the three
207	     attributes mentioned above (raid_disks, chunk_size, layout) can
208	     potentially have 2 values, an old and a new value.  If these
209	     values differ, reading the attribute returns
210	        new (old)
211	     and writing will effect the 'new' value, leaving the 'old'
212	     unchanged.
214	  component_size
215	     For arrays with data redundancy (i.e. not raid0, linear, faulty,
216	     multipath), all components must be the same size - or at least
217	     there must a size that they all provide space for.  This is a key
218	     part or the geometry of the array.  It is measured in sectors
219	     and can be read from here.  Writing to this value may resize
220	     the array if the personality supports it (raid1, raid5, raid6),
221	     and if the component drives are large enough.
223	  metadata_version
224	     This indicates the format that is being used to record metadata
225	     about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
226	     1.2 (newer format in varying locations) or "none" indicating that
227	     the kernel isn't managing metadata at all.
228	     Alternately it can be "external:" followed by a string which
229	     is set by user-space.  This indicates that metadata is managed
230	     by a user-space program.  Any device failure or other event that
231	     requires a metadata update will cause array activity to be
232	     suspended until the event is acknowledged.
234	  resync_start
235	     The point at which resync should start.  If no resync is needed,
236	     this will be a very large number (or 'none' since 2.6.30-rc1).  At
237	     array creation it will default to 0, though starting the array as
238	     'clean' will set it much larger.
240	   new_dev
241	     This file can be written but not read.  The value written should
242	     be a block device number as major:minor.  e.g. 8:0
243	     This will cause that device to be attached to the array, if it is
244	     available.  It will then appear at md/dev-XXX (depending on the
245	     name of the device) and further configuration is then possible.
247	   safe_mode_delay
248	     When an md array has seen no write requests for a certain period
249	     of time, it will be marked as 'clean'.  When another write
250	     request arrives, the array is marked as 'dirty' before the write
251	     commences.  This is known as 'safe_mode'.
252	     The 'certain period' is controlled by this file which stores the
253	     period as a number of seconds.  The default is 200msec (0.200).
254	     Writing a value of 0 disables safemode.
256	   array_state
257	     This file contains a single word which describes the current
258	     state of the array.  In many cases, the state can be set by
259	     writing the word for the desired state, however some states
260	     cannot be explicitly set, and some transitions are not allowed.
262	     Select/poll works on this file.  All changes except between
263	     	active_idle and active (which can be frequent and are not
264		very interesting) are notified.  active->active_idle is
265		reported if the metadata is externally managed.
267	     clear
268	         No devices, no size, no level
269	         Writing is equivalent to STOP_ARRAY ioctl
270	     inactive
271	         May have some settings, but array is not active
272	            all IO results in error
273	         When written, doesn't tear down array, but just stops it
274	     suspended (not supported yet)
275	         All IO requests will block. The array can be reconfigured.
276	         Writing this, if accepted, will block until array is quiessent
277	     readonly
278	         no resync can happen.  no superblocks get written.
279	         write requests fail
280	     read-auto
281	         like readonly, but behaves like 'clean' on a write request.
283	     clean - no pending writes, but otherwise active.
284	         When written to inactive array, starts without resync
285	         If a write request arrives then
286	           if metadata is known, mark 'dirty' and switch to 'active'.
287	           if not known, block and switch to write-pending
288	         If written to an active array that has pending writes, then fails.
289	     active
290	         fully active: IO and resync can be happening.
291	         When written to inactive array, starts with resync
293	     write-pending
294	         clean, but writes are blocked waiting for 'active' to be written.
296	     active-idle
297	         like active, but no writes have been seen for a while (safe_mode_delay).
299	  bitmap/location
300	     This indicates where the write-intent bitmap for the array is
301	     stored.
302	     It can be one of "none", "file" or "[+-]N".
303	     "file" may later be extended to "file:/file/name"
304	     "[+-]N" means that many sectors from the start of the metadata.
305	       This is replicated on all devices.  For arrays with externally
306	       managed metadata, the offset is from the beginning of the
307	       device.
308	  bitmap/chunksize
309	     The size, in bytes, of the chunk which will be represented by a
310	     single bit.  For RAID456, it is a portion of an individual
311	     device. For RAID10, it is a portion of the array.  For RAID1, it
312	     is both (they come to the same thing).
313	  bitmap/time_base
314	     The time, in seconds, between looking for bits in the bitmap to
315	     be cleared. In the current implementation, a bit will be cleared
316	     between 2 and 3 times "time_base" after all the covered blocks
317	     are known to be in-sync.
318	  bitmap/backlog
319	     When write-mostly devices are active in a RAID1, write requests
320	     to those devices proceed in the background - the filesystem (or
321	     other user of the device) does not have to wait for them.
322	     'backlog' sets a limit on the number of concurrent background
323	     writes.  If there are more than this, new writes will by
324	     synchronous.
325	  bitmap/metadata
326	     This can be either 'internal' or 'external'.
327	     'internal' is the default and means the metadata for the bitmap
328	     is stored in the first 256 bytes of the allocated space and is
329	     managed by the md module.
330	     'external' means that bitmap metadata is managed externally to
331	     the kernel (i.e. by some userspace program)
332	  bitmap/can_clear
333	     This is either 'true' or 'false'.  If 'true', then bits in the
334	     bitmap will be cleared when the corresponding blocks are thought
335	     to be in-sync.  If 'false', bits will never be cleared.
336	     This is automatically set to 'false' if a write happens on a
337	     degraded array, or if the array becomes degraded during a write.
338	     When metadata is managed externally, it should be set to true
339	     once the array becomes non-degraded, and this fact has been
340	     recorded in the metadata.
345	As component devices are added to an md array, they appear in the 'md'
346	directory as new directories named
347	      dev-XXX
348	where XXX is a name that the kernel knows for the device, e.g. hdb1.
349	Each directory contains:
351	      block
352	        a symlink to the block device in /sys/block, e.g.
353		     /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
355	      super
356	        A file containing an image of the superblock read from, or
357	        written to, that device.
359	      state
360		A file recording the current state of the device in the array
361		which can be a comma separated list of
362		      faulty   - device has been kicked from active use due to
363				 a detected fault, or it has unacknowledged bad
364				 blocks
365		      in_sync  - device is a fully in-sync member of the array
366		      writemostly - device will only be subject to read
367				 requests if there are no other options.
368				 This applies only to raid1 arrays.
369		      blocked  - device has failed, and the failure hasn't been
370				 acknowledged yet by the metadata handler.
371				 Writes that would write to this device if
372				 it were not faulty are blocked.
373		      spare    - device is working, but not a full member.
374				 This includes spares that are in the process
375				 of being recovered to
376		      write_error - device has ever seen a write error.
377		      want_replacement - device is (mostly) working but probably
378				 should be replaced, either due to errors or
379				 due to user request.
380		      replacement - device is a replacement for another active
381				 device with same raid_disk.
384		This list may grow in future.
385		This can be written to.
386		Writing "faulty"  simulates a failure on the device.
387		Writing "remove" removes the device from the array.
388		Writing "writemostly" sets the writemostly flag.
389		Writing "-writemostly" clears the writemostly flag.
390		Writing "blocked" sets the "blocked" flag.
391		Writing "-blocked" clears the "blocked" flags and allows writes
392			to complete and possibly simulates an error.
393		Writing "in_sync" sets the in_sync flag.
394		Writing "write_error" sets writeerrorseen flag.
395		Writing "-write_error" clears writeerrorseen flag.
396		Writing "want_replacement" is allowed at any time except to a
397			replacement device or a spare.  It sets the flag.
398		Writing "-want_replacement" is allowed at any time.  It clears
399			the flag.
400		Writing "replacement" or "-replacement" is only allowed before
401			starting the array.  It sets or clears the flag.
404		This file responds to select/poll. Any change to 'faulty'
405		or 'blocked' causes an event.
407	      errors
408		An approximate count of read errors that have been detected on
409		this device but have not caused the device to be evicted from
410		the array (either because they were corrected or because they
411		happened while the array was read-only).  When using version-1
412		metadata, this value persists across restarts of the array.
414		This value can be written while assembling an array thus
415		providing an ongoing count for arrays with metadata managed by
416		userspace.
418	      slot
419	        This gives the role that the device has in the array.  It will
420		either be 'none' if the device is not active in the array
421	        (i.e. is a spare or has failed) or an integer less than the
422		'raid_disks' number for the array indicating which position
423		it currently fills.  This can only be set while assembling an
424		array.  A device for which this is set is assumed to be working.
426	      offset
427	        This gives the location in the device (in sectors from the
428	        start) where data from the array will be stored.  Any part of
429	        the device before this offset is not touched, unless it is
430	        used for storing metadata (Formats 1.1 and 1.2).
432	      size
433	        The amount of the device, after the offset, that can be used
434	        for storage of data.  This will normally be the same as the
435		component_size.  This can be written while assembling an
436	        array.  If a value less than the current component_size is
437	        written, it will be rejected.
439	      recovery_start
440	        When the device is not 'in_sync', this records the number of
441		sectors from the start of the device which are known to be
442		correct.  This is normally zero, but during a recovery
443		operation it will steadily increase, and if the recovery is
444		interrupted, restoring this value can cause recovery to
445		avoid repeating the earlier blocks.  With v1.x metadata, this
446		value is saved and restored automatically.
448		This can be set whenever the device is not an active member of
449		the array, either before the array is activated, or before
450		the 'slot' is set.
452		Setting this to 'none' is equivalent to setting 'in_sync'.
453		Setting to any other value also clears the 'in_sync' flag.
455	      bad_blocks
456		This gives the list of all known bad blocks in the form of
457		start address and length (in sectors respectively). If output
458		is too big to fit in a page, it will be truncated. Writing
459		"sector length" to this file adds new acknowledged (i.e.
460		recorded to disk safely) bad blocks.
462	      unacknowledged_bad_blocks
463		This gives the list of known-but-not-yet-saved-to-disk bad
464		blocks in the same form of 'bad_blocks'. If output is too big
465		to fit in a page, it will be truncated. Writing to this file
466		adds bad blocks without acknowledging them. This is largely
467		for testing.
471	An active md device will also contain an entry for each active device
472	in the array.  These are named
474	    rdNN
476	where 'NN' is the position in the array, starting from 0.
477	So for a 3 drive array there will be rd0, rd1, rd2.
478	These are symbolic links to the appropriate 'dev-XXX' entry.
479	Thus, for example,
480	       cat /sys/block/md*/md/rd*/state
481	will show 'in_sync' on every line.
485	Active md devices for levels that support data redundancy (1,4,5,6,10)
486	also have
488	   sync_action
489	     a text file that can be used to monitor and control the rebuild
490	     process.  It contains one word which can be one of:
491	       resync        - redundancy is being recalculated after unclean
492	                       shutdown or creation
493	       recover       - a hot spare is being built to replace a
494	                       failed/missing device
495	       idle          - nothing is happening
496	       check         - A full check of redundancy was requested and is
497	                       happening.  This reads all blocks and checks
498	                       them. A repair may also happen for some raid
499	                       levels.
500	       repair        - A full check and repair is happening.  This is
501	                       similar to 'resync', but was requested by the
502	                       user, and the write-intent bitmap is NOT used to
503			       optimise the process.
505	      This file is writable, and each of the strings that could be
506	      read are meaningful for writing.
508	       'idle' will stop an active resync/recovery etc.  There is no
509	           guarantee that another resync/recovery may not be automatically
510		   started again, though some event will be needed to trigger
511	           this.
512		'resync' or 'recovery' can be used to restart the
513	           corresponding operation if it was stopped with 'idle'.
514		'check' and 'repair' will start the appropriate process
515	           providing the current state is 'idle'.
517	      This file responds to select/poll.  Any important change in the value
518	      triggers a poll event.  Sometimes the value will briefly be
519	      "recover" if a recovery seems to be needed, but cannot be
520	      achieved. In that case, the transition to "recover" isn't
521	      notified, but the transition away is.
523	   degraded
524	      This contains a count of the number of devices by which the
525	      arrays is degraded.  So an optimal array will show '0'.  A
526	      single failed/missing drive will show '1', etc.
527	      This file responds to select/poll, any increase or decrease
528	      in the count of missing devices will trigger an event.
530	   mismatch_count
531	      When performing 'check' and 'repair', and possibly when
532	      performing 'resync', md will count the number of errors that are
533	      found.  The count in 'mismatch_cnt' is the number of sectors
534	      that were re-written, or (for 'check') would have been
535	      re-written.  As most raid levels work in units of pages rather
536	      than sectors, this may be larger than the number of actual errors
537	      by a factor of the number of sectors in a page.
539	   bitmap_set_bits
540	      If the array has a write-intent bitmap, then writing to this
541	      attribute can set bits in the bitmap, indicating that a resync
542	      would need to check the corresponding blocks. Either individual
543	      numbers or start-end pairs can be written.  Multiple numbers
544	      can be separated by a space.
545	      Note that the numbers are 'bit' numbers, not 'block' numbers.
546	      They should be scaled by the bitmap_chunksize.
548	   sync_speed_min
549	   sync_speed_max
550	     This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
551	     however they only apply to the particular array.
552	     If no value has been written to these, or if the word 'system'
553	     is written, then the system-wide value is used.  If a value,
554	     in kibibytes-per-second is written, then it is used.
555	     When the files are read, they show the currently active value
556	     followed by "(local)" or "(system)" depending on whether it is
557	     a locally set or system-wide value.
559	   sync_completed
560	     This shows the number of sectors that have been completed of
561	     whatever the current sync_action is, followed by the number of
562	     sectors in total that could need to be processed.  The two
563	     numbers are separated by a '/'  thus effectively showing one
564	     value, a fraction of the process that is complete.
565	     A 'select' on this attribute will return when resync completes,
566	     when it reaches the current sync_max (below) and possibly at
567	     other times.
569	   sync_speed
570	     This shows the current actual speed, in K/sec, of the current
571	     sync_action.  It is averaged over the last 30 seconds.
573	   suspend_lo
574	   suspend_hi
575	     The two values, given as numbers of sectors, indicate a range
576	     within the array where IO will be blocked.  This is currently
577	     only supported for raid4/5/6.
579	   sync_min
580	   sync_max
581	     The two values, given as numbers of sectors, indicate a range
582	     within the array where 'check'/'repair' will operate. Must be
583	     a multiple of chunk_size. When it reaches "sync_max" it will
584	     pause, rather than complete.
585	     You can use 'select' or 'poll' on "sync_completed" to wait for
586	     that number to reach sync_max.  Then you can either increase
587	     "sync_max", or can write 'idle' to "sync_action".
589	     The value of 'max' for "sync_max" effectively disables the limit.
590	     When a resync is active, the value can only ever be increased,
591	     never decreased.
592	     The value of '0' is the minimum for "sync_min".
596	Each active md device may also have attributes specific to the
597	personality module that manages it.
598	These are specific to the implementation of the module and could
599	change substantially if the implementation changes.
601	These currently include
603	  stripe_cache_size  (currently raid5 only)
604	      number of entries in the stripe cache.  This is writable, but
605	      there are upper and lower limits (32768, 17).  Default is 256.
606	  strip_cache_active (currently raid5 only)
607	      number of active entries in the stripe cache
608	  preread_bypass_threshold (currently raid5 only)
609	      number of times a stripe requiring preread will be bypassed by
610	      a stripe that does not require preread.  For fairness defaults
611	      to 1.  Setting this to 0 disables bypass accounting and
612	      requires preread stripes to wait until all full-width stripe-
613	      writes are complete.  Valid values are 0 to stripe_cache_size.
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