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Documentation / device-mapper / dm-raid.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

1	dm-raid
2	=======
3	
4	The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.
5	It allows the MD RAID drivers to be accessed using a device-mapper
6	interface.
7	
8	
9	Mapping Table Interface
10	-----------------------
11	The target is named "raid" and it accepts the following parameters:
12	
13	  <raid_type> <#raid_params> <raid_params> \
14	    <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]
15	
16	<raid_type>:
17	  raid1		RAID1 mirroring
18	  raid4		RAID4 dedicated parity disk
19	  raid5_la	RAID5 left asymmetric
20			- rotating parity 0 with data continuation
21	  raid5_ra	RAID5 right asymmetric
22			- rotating parity N with data continuation
23	  raid5_ls	RAID5 left symmetric
24			- rotating parity 0 with data restart
25	  raid5_rs 	RAID5 right symmetric
26			- rotating parity N with data restart
27	  raid6_zr	RAID6 zero restart
28			- rotating parity zero (left-to-right) with data restart
29	  raid6_nr	RAID6 N restart
30			- rotating parity N (right-to-left) with data restart
31	  raid6_nc	RAID6 N continue
32			- rotating parity N (right-to-left) with data continuation
33	  raid10        Various RAID10 inspired algorithms chosen by additional params
34			- RAID10: Striped Mirrors (aka 'Striping on top of mirrors')
35			- RAID1E: Integrated Adjacent Stripe Mirroring
36			- RAID1E: Integrated Offset Stripe Mirroring
37			-  and other similar RAID10 variants
38	
39	  Reference: Chapter 4 of
40	  http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
41	
42	<#raid_params>: The number of parameters that follow.
43	
44	<raid_params> consists of
45	    Mandatory parameters:
46	        <chunk_size>: Chunk size in sectors.  This parameter is often known as
47			      "stripe size".  It is the only mandatory parameter and
48			      is placed first.
49	
50	    followed by optional parameters (in any order):
51		[sync|nosync]   Force or prevent RAID initialization.
52	
53		[rebuild <idx>]	Rebuild drive number 'idx' (first drive is 0).
54	
55		[daemon_sleep <ms>]
56			Interval between runs of the bitmap daemon that
57			clear bits.  A longer interval means less bitmap I/O but
58			resyncing after a failure is likely to take longer.
59	
60		[min_recovery_rate <kB/sec/disk>]  Throttle RAID initialization
61		[max_recovery_rate <kB/sec/disk>]  Throttle RAID initialization
62		[write_mostly <idx>]		   Mark drive index 'idx' write-mostly.
63		[max_write_behind <sectors>]       See '--write-behind=' (man mdadm)
64		[stripe_cache <sectors>]           Stripe cache size (RAID 4/5/6 only)
65		[region_size <sectors>]
66			The region_size multiplied by the number of regions is the
67			logical size of the array.  The bitmap records the device
68			synchronisation state for each region.
69	
70	        [raid10_copies   <# copies>]
71	        [raid10_format   <near|far|offset>]
72			These two options are used to alter the default layout of
73			a RAID10 configuration.  The number of copies is can be
74			specified, but the default is 2.  There are also three
75			variations to how the copies are laid down - the default
76			is "near".  Near copies are what most people think of with
77			respect to mirroring.  If these options are left unspecified,
78			or 'raid10_copies 2' and/or 'raid10_format near' are given,
79			then the layouts for 2, 3 and 4 devices	are:
80			2 drives         3 drives          4 drives
81			--------         ----------        --------------
82			A1  A1           A1  A1  A2        A1  A1  A2  A2
83			A2  A2           A2  A3  A3        A3  A3  A4  A4
84			A3  A3           A4  A4  A5        A5  A5  A6  A6
85			A4  A4           A5  A6  A6        A7  A7  A8  A8
86			..  ..           ..  ..  ..        ..  ..  ..  ..
87			The 2-device layout is equivalent 2-way RAID1.  The 4-device
88			layout is what a traditional RAID10 would look like.  The
89			3-device layout is what might be called a 'RAID1E - Integrated
90			Adjacent Stripe Mirroring'.
91	
92			If 'raid10_copies 2' and 'raid10_format far', then the layouts
93			for 2, 3 and 4 devices are:
94			2 drives             3 drives             4 drives
95			--------             --------------       --------------------
96			A1  A2               A1   A2   A3         A1   A2   A3   A4
97			A3  A4               A4   A5   A6         A5   A6   A7   A8
98			A5  A6               A7   A8   A9         A9   A10  A11  A12
99			..  ..               ..   ..   ..         ..   ..   ..   ..
100			A2  A1               A3   A1   A2         A2   A1   A4   A3
101			A4  A3               A6   A4   A5         A6   A5   A8   A7
102			A6  A5               A9   A7   A8         A10  A9   A12  A11
103			..  ..               ..   ..   ..         ..   ..   ..   ..
104	
105			If 'raid10_copies 2' and 'raid10_format offset', then the
106			layouts for 2, 3 and 4 devices are:
107			2 drives       3 drives           4 drives
108			--------       ------------       -----------------
109			A1  A2         A1  A2  A3         A1  A2  A3  A4
110			A2  A1         A3  A1  A2         A2  A1  A4  A3
111			A3  A4         A4  A5  A6         A5  A6  A7  A8
112			A4  A3         A6  A4  A5         A6  A5  A8  A7
113			A5  A6         A7  A8  A9         A9  A10 A11 A12
114			A6  A5         A9  A7  A8         A10 A9  A12 A11
115			..  ..         ..  ..  ..         ..  ..  ..  ..
116			Here we see layouts closely akin to 'RAID1E - Integrated
117			Offset Stripe Mirroring'.
118	
119	<#raid_devs>: The number of devices composing the array.
120		Each device consists of two entries.  The first is the device
121		containing the metadata (if any); the second is the one containing the
122		data.
123	
124		If a drive has failed or is missing at creation time, a '-' can be
125		given for both the metadata and data drives for a given position.
126	
127	
128	Example Tables
129	--------------
130	# RAID4 - 4 data drives, 1 parity (no metadata devices)
131	# No metadata devices specified to hold superblock/bitmap info
132	# Chunk size of 1MiB
133	# (Lines separated for easy reading)
134	
135	0 1960893648 raid \
136	        raid4 1 2048 \
137	        5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81
138	
139	# RAID4 - 4 data drives, 1 parity (with metadata devices)
140	# Chunk size of 1MiB, force RAID initialization,
141	#       min recovery rate at 20 kiB/sec/disk
142	
143	0 1960893648 raid \
144	        raid4 4 2048 sync min_recovery_rate 20 \
145	        5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82
146	
147	
148	Status Output
149	-------------
150	'dmsetup table' displays the table used to construct the mapping.
151	The optional parameters are always printed in the order listed
152	above with "sync" or "nosync" always output ahead of the other
153	arguments, regardless of the order used when originally loading the table.
154	Arguments that can be repeated are ordered by value.
155	
156	
157	'dmsetup status' yields information on the state and health of the array.
158	The output is as follows (normally a single line, but expanded here for
159	clarity):
160	1: <s> <l> raid \
161	2:      <raid_type> <#devices> <health_chars> \
162	3:      <sync_ratio> <sync_action> <mismatch_cnt>
163	
164	Line 1 is the standard output produced by device-mapper.
165	Line 2 & 3 are produced by the raid target and are best explained by example:
166	        0 1960893648 raid raid4 5 AAAAA 2/490221568 init 0
167	Here we can see the RAID type is raid4, there are 5 devices - all of
168	which are 'A'live, and the array is 2/490221568 complete with its initial
169	recovery.  Here is a fuller description of the individual fields:
170		<raid_type>     Same as the <raid_type> used to create the array.
171		<health_chars>  One char for each device, indicating: 'A' = alive and
172				in-sync, 'a' = alive but not in-sync, 'D' = dead/failed.
173		<sync_ratio>    The ratio indicating how much of the array has undergone
174				the process described by 'sync_action'.  If the
175				'sync_action' is "check" or "repair", then the process
176				of "resync" or "recover" can be considered complete.
177		<sync_action>   One of the following possible states:
178				idle    - No synchronization action is being performed.
179				frozen  - The current action has been halted.
180				resync  - Array is undergoing its initial synchronization
181					  or is resynchronizing after an unclean shutdown
182					  (possibly aided by a bitmap).
183				recover - A device in the array is being rebuilt or
184					  replaced.
185				check   - A user-initiated full check of the array is
186					  being performed.  All blocks are read and
187					  checked for consistency.  The number of
188					  discrepancies found are recorded in
189					  <mismatch_cnt>.  No changes are made to the
190					  array by this action.
191				repair  - The same as "check", but discrepancies are
192					  corrected.
193				reshape - The array is undergoing a reshape.
194		<mismatch_cnt>  The number of discrepancies found between mirror copies
195				in RAID1/10 or wrong parity values found in RAID4/5/6.
196				This value is valid only after a "check" of the array
197				is performed.  A healthy array has a 'mismatch_cnt' of 0.
198	
199	Message Interface
200	-----------------
201	The dm-raid target will accept certain actions through the 'message' interface.
202	('man dmsetup' for more information on the message interface.)  These actions
203	include:
204		"idle"   - Halt the current sync action.
205		"frozen" - Freeze the current sync action.
206		"resync" - Initiate/continue a resync.
207		"recover"- Initiate/continue a recover process.
208		"check"  - Initiate a check (i.e. a "scrub") of the array.
209		"repair" - Initiate a repair of the array.
210		"reshape"- Currently unsupported (-EINVAL).
211	
212	Version History
213	---------------
214	1.0.0	Initial version.  Support for RAID 4/5/6
215	1.1.0	Added support for RAID 1
216	1.2.0	Handle creation of arrays that contain failed devices.
217	1.3.0	Added support for RAID 10
218	1.3.1	Allow device replacement/rebuild for RAID 10
219	1.3.2   Fix/improve redundancy checking for RAID10
220	1.4.0	Non-functional change.  Removes arg from mapping function.
221	1.4.1   RAID10 fix redundancy validation checks (commit 55ebbb5).
222	1.4.2   Add RAID10 "far" and "offset" algorithm support.
223	1.5.0   Add message interface to allow manipulation of the sync_action.
224		New status (STATUSTYPE_INFO) fields: sync_action and mismatch_cnt.
225	1.5.1   Add ability to restore transiently failed devices on resume.
226	1.5.2   'mismatch_cnt' is zero unless [last_]sync_action is "check".
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