[net-next-2.6.git] / Documentation / md.txt

Tools that manage md devices can be found at
   http://www.<country>.kernel.org/pub/linux/utils/raid/....


Boot time assembly of RAID arrays
---------------------------------

You can boot with your md device with the following kernel command
lines:

for old raid arrays without persistent superblocks:
  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn

for raid arrays with persistent superblocks
  md=<md device no.>,dev0,dev1,...,devn
or, to assemble a partitionable array:
  md=d<md device no.>,dev0,dev1,...,devn
  
md device no. = the number of the md device ... 
              0 means md0, 
	      1 md1,
	      2 md2,
	      3 md3,
	      4 md4

raid level = -1 linear mode
              0 striped mode
	      other modes are only supported with persistent super blocks

chunk size factor = (raid-0 and raid-1 only)
              Set  the chunk size as 4k << n.
	      
fault level = totally ignored
			    
dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
			    
A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this:

e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro


Boot time autodetection of RAID arrays
--------------------------------------

When md is compiled into the kernel (not as module), partitions of
type 0xfd are scanned and automatically assembled into RAID arrays.
This autodetection may be suppressed with the kernel parameter
"raid=noautodetect".  As of kernel 2.6.9, only drives with a type 0
superblock can be autodetected and run at boot time.

The kernel parameter "raid=partitionable" (or "raid=part") means
that all auto-detected arrays are assembled as partitionable.


Superblock formats
------------------

The md driver can support a variety of different superblock formats.
Currently, it supports superblock formats "0.90.0" and the "md-1" format
introduced in the 2.5 development series.

The kernel will autodetect which format superblock is being used.

Superblock format '0' is treated differently to others for legacy
reasons - it is the original superblock format.


General Rules - apply for all superblock formats
------------------------------------------------

An array is 'created' by writing appropriate superblocks to all
devices.

It is 'assembled' by associating each of these devices with an
particular md virtual device.  Once it is completely assembled, it can
be accessed.

An array should be created by a user-space tool.  This will write
superblocks to all devices.  It will usually mark the array as
'unclean', or with some devices missing so that the kernel md driver
can create appropriate redundancy (copying in raid1, parity
calculation in raid4/5).

When an array is assembled, it is first initialized with the
SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
version number.  The major version number selects which superblock
format is to be used.  The minor number might be used to tune handling
of the format, such as suggesting where on each device to look for the
superblock.

Then each device is added using the ADD_NEW_DISK ioctl.  This
provides, in particular, a major and minor number identifying the
device to add.

The array is started with the RUN_ARRAY ioctl.

Once started, new devices can be added.  They should have an
appropriate superblock written to them, and then passed be in with
ADD_NEW_DISK.

Devices that have failed or are not yet active can be detached from an
array using HOT_REMOVE_DISK.


Specific Rules that apply to format-0 super block arrays, and
       arrays with no superblock (non-persistent).
-------------------------------------------------------------

An array can be 'created' by describing the array (level, chunksize
etc) in a SET_ARRAY_INFO ioctl.  This must has major_version==0 and
raid_disks != 0.

Then uninitialized devices can be added with ADD_NEW_DISK.  The
structure passed to ADD_NEW_DISK must specify the state of the device
and it's role in the array.

Once started with RUN_ARRAY, uninitialized spares can be added with
HOT_ADD_DISK.


MD devices in sysfs
-------------------
md devices appear in sysfs (/sys) as regular block devices,
e.g.
   /sys/block/md0

Each 'md' device will contain a subdirectory called 'md' which
contains further md-specific information about the device.

All md devices contain:
  level
     a text file indicating the 'raid level'.  This may be a standard
     numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
     other name such as "linear" or "multipath".
     If no raid level has been set yet (array is still being
     assembled), this file will be empty.

  raid_disks
     a text file with a simple number indicating the number of devices
     in a fully functional array.  If this is not yet known, the file
     will be empty.  If an array is being resized (not currently
     possible) this will contain the larger of the old and new sizes.

As component devices are added to an md array, they appear in the 'md'
directory as new directories named
      dev-XXX
where XXX is a name that the kernel knows for the device, e.g. hdb1.
Each directory contains:

      block
        a symlink to the block device in /sys/block, e.g.
	     /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1

      super
        A file containing an image of the superblock read from, or
        written to, that device.

      state
        A file recording the current state of the device in the array
	which can be a comma separated list of
	      faulty   - device has been kicked from active use due to
                         a detected fault
	      in_sync  - device is a fully in-sync member of the array
	      spare    - device is working, but not a full member.
			 This includes spares that are in the process
			 of being recoverred to
	This list make grow in future.


An active md device will also contain and entry for each active device
in the array.  These are named

    rdNN

where 'NN' is the possition in the array, starting from 0.
So for a 3 drive array there will be rd0, rd1, rd2.
These are symbolic links to the appropriate 'dev-XXX' entry.
Thus, for example,
       cat /sys/block/md*/md/rd*/state
will show 'in_sync' on every line.


Active md devices for levels that support data redundancy (1,4,5,6)
also have

   sync_action
     a text file that can be used to monitor and control the rebuild
     process.  It contains one word which can be one of:
       resync        - redundancy is being recalculated after unclean
                       shutdown or creation
       recover       - a hot spare is being built to replace a
                       failed/missing device
       idle          - nothing is happening
       check         - A full check of redundancy was requested and is
                       happening.  This reads all block and checks
                       them. A repair may also happen for some raid
                       levels.
       repair        - A full check and repair is happening.  This is
                       similar to 'resync', but was requested by the
                       user, and the write-intent bitmap is NOT used to
		       optimise the process.

      This file is writable, and each of the strings that could be
      read are meaningful for writing.

       'idle' will stop an active resync/recovery etc.  There is no
           guarantee that another resync/recovery may not be automatically
	   started again, though some event will be needed to trigger
           this.
	'resync' or 'recovery' can be used to restart the
           corresponding operation if it was stopped with 'idle'.
	'check' and 'repair' will start the appropriate process
           providing the current state is 'idle'.

   mismatch_count
      When performing 'check' and 'repair', and possibly when
      performing 'resync', md will count the number of errors that are
      found.  The count in 'mismatch_cnt' is the number of sectors
      that were re-written, or (for 'check') would have been
      re-written.  As most raid levels work in units of pages rather
      than sectors, this my be larger than the number of actual errors
      by a factor of the number of sectors in a page.

Each active md device may also have attributes specific to the
personality module that manages it.
These are specific to the implementation of the module and could
change substantially if the implementation changes.

These currently include

  stripe_cache_size  (currently raid5 only)
      number of entries in the stripe cache.  This is writable, but
      there are upper and lower limits (32768, 16).  Default is 128.
  strip_cache_active (currently raid5 only)
      number of active entries in the stripe cache
Commit	Line	Data
1da177e4 LT	1	Tools that manage md devices can be found at
	2	http://www.<country>.kernel.org/pub/linux/utils/raid/....
	3
	4
	5	Boot time assembly of RAID arrays
	6	---------------------------------
	7
	8	You can boot with your md device with the following kernel command
	9	lines:
	10
	11	for old raid arrays without persistent superblocks:
	12	md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
	13
	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
	18
	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
	25
	26	raid level = -1 linear mode
	27	0 striped mode
	28	other modes are only supported with persistent super blocks
	29
	30	chunk size factor = (raid-0 and raid-1 only)
	31	Set the chunk size as 4k << n.
	32
	33	fault level = totally ignored
	34
	35	dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
	36
	37	A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this:
	38
	39	e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
	40
	41
	42	Boot time autodetection of RAID arrays
	43	--------------------------------------
	44
	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.
	50
	51	The kernel parameter "raid=partitionable" (or "raid=part") means
	52	that all auto-detected arrays are assembled as partitionable.
	53
	54
	55	Superblock formats
	56	------------------
	57
	58	The md driver can support a variety of different superblock formats.
	59	Currently, it supports superblock formats "0.90.0" and the "md-1" format
	60	introduced in the 2.5 development series.
	61
	62	The kernel will autodetect which format superblock is being used.
	63
	64	Superblock format '0' is treated differently to others for legacy
65	reasons - it is the original superblock format.
66
67
68	General Rules - apply for all superblock formats
69	------------------------------------------------
70
71	An array is 'created' by writing appropriate superblocks to all
72	devices.
73
74	It is 'assembled' by associating each of these devices with an
75	particular md virtual device. Once it is completely assembled, it can
76	be accessed.
77
78	An array should be created by a user-space tool. This will write
79	superblocks to all devices. It will usually mark the array as
80	'unclean', or with some devices missing so that the kernel md driver
81	can create appropriate redundancy (copying in raid1, parity
82	calculation in raid4/5).
83
84	When an array is assembled, it is first initialized with the
85	SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor
86	version number. The major version number selects which superblock
87	format is to be used. The minor number might be used to tune handling
88	of the format, such as suggesting where on each device to look for the
89	superblock.
90
91	Then each device is added using the ADD_NEW_DISK ioctl. This
92	provides, in particular, a major and minor number identifying the
93	device to add.
94
95	The array is started with the RUN_ARRAY ioctl.
96
97	Once started, new devices can be added. They should have an
98	appropriate superblock written to them, and then passed be in with
99	ADD_NEW_DISK.
100
101	Devices that have failed or are not yet active can be detached from an
102	array using HOT_REMOVE_DISK.
103
104
105	Specific Rules that apply to format-0 super block arrays, and
106	arrays with no superblock (non-persistent).
107	-------------------------------------------------------------
108
109	An array can be 'created' by describing the array (level, chunksize
110	etc) in a SET_ARRAY_INFO ioctl. This must has major_version==0 and
111	raid_disks != 0.
112
113	Then uninitialized devices can be added with ADD_NEW_DISK. The
114	structure passed to ADD_NEW_DISK must specify the state of the device
115	and it's role in the array.
116
117	Once started with RUN_ARRAY, uninitialized spares can be added with
118	HOT_ADD_DISK.
bb636547 N	119
	120
	121
	122	MD devices in sysfs
	123	-------------------
	124	md devices appear in sysfs (/sys) as regular block devices,
	125	e.g.
	126	/sys/block/md0
	127
	128	Each 'md' device will contain a subdirectory called 'md' which
	129	contains further md-specific information about the device.
	130
	131	All md devices contain:
	132	level
	133	a text file indicating the 'raid level'. This may be a standard
	134	numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
	135	other name such as "linear" or "multipath".
	136	If no raid level has been set yet (array is still being
	137	assembled), this file will be empty.
	138
	139	raid_disks
	140	a text file with a simple number indicating the number of devices
	141	in a fully functional array. If this is not yet known, the file
	142	will be empty. If an array is being resized (not currently
	143	possible) this will contain the larger of the old and new sizes.
	144
	145	As component devices are added to an md array, they appear in the 'md'
	146	directory as new directories named
	147	dev-XXX
	148	where XXX is a name that the kernel knows for the device, e.g. hdb1.
	149	Each directory contains:
	150
	151	block
	152	a symlink to the block device in /sys/block, e.g.
	153	/sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
	154
	155	super
	156	A file containing an image of the superblock read from, or
	157	written to, that device.
	158
	159	state
	160	A file recording the current state of the device in the array
	161	which can be a comma separated list of
	162	faulty - device has been kicked from active use due to
	163	a detected fault
	164	in_sync - device is a fully in-sync member of the array
	165	spare - device is working, but not a full member.
	166	This includes spares that are in the process
	167	of being recoverred to
	168	This list make grow in future.
	169
	170
	171	An active md device will also contain and entry for each active device
	172	in the array. These are named
	173
	174	rdNN
	175
	176	where 'NN' is the possition in the array, starting from 0.
	177	So for a 3 drive array there will be rd0, rd1, rd2.
	178	These are symbolic links to the appropriate 'dev-XXX' entry.
	179	Thus, for example,
	180	cat /sys/block/md/md/rd/state
	181	will show 'in_sync' on every line.
	182
183
184
185	Active md devices for levels that support data redundancy (1,4,5,6)
186	also have
187
188	sync_action
189	a text file that can be used to monitor and control the rebuild
190	process. It contains one word which can be one of:
191	resync - redundancy is being recalculated after unclean
192	shutdown or creation
193	recover - a hot spare is being built to replace a
194	failed/missing device
195	idle - nothing is happening
196	check - A full check of redundancy was requested and is
197	happening. This reads all block and checks
198	them. A repair may also happen for some raid
199	levels.
200	repair - A full check and repair is happening. This is
201	similar to 'resync', but was requested by the
202	user, and the write-intent bitmap is NOT used to
203	optimise the process.
204
205	This file is writable, and each of the strings that could be
206	read are meaningful for writing.
207
208	'idle' will stop an active resync/recovery etc. There is no
209	guarantee that another resync/recovery may not be automatically
210	started again, though some event will be needed to trigger
211	this.
212	'resync' or 'recovery' can be used to restart the
213	corresponding operation if it was stopped with 'idle'.
214	'check' and 'repair' will start the appropriate process
215	providing the current state is 'idle'.
216
217	mismatch_count
218	When performing 'check' and 'repair', and possibly when
219	performing 'resync', md will count the number of errors that are
220	found. The count in 'mismatch_cnt' is the number of sectors
221	that were re-written, or (for 'check') would have been
222	re-written. As most raid levels work in units of pages rather
223	than sectors, this my be larger than the number of actual errors
224	by a factor of the number of sectors in a page.
225
226	Each active md device may also have attributes specific to the
227	personality module that manages it.
228	These are specific to the implementation of the module and could
229	change substantially if the implementation changes.
230
231	These currently include
232
233	stripe_cache_size (currently raid5 only)
234	number of entries in the stripe cache. This is writable, but
235	there are upper and lower limits (32768, 16). Default is 128.
236	strip_cache_active (currently raid5 only)
237	number of active entries in the stripe cache