| blob | df2d636b60880cb36c105f68c953d60e50110c77 |
1 dm-raid
2 =======
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.
9 Mapping Table Interface
10 -----------------------
11 The target is named "raid" and it accepts the following parameters:
13 <raid_type> <#raid_params> <raid_params> \
14 <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]
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
39 Reference: Chapter 4 of
40 http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
42 <#raid_params>: The number of parameters that follow.
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.
50 followed by optional parameters (in any order):
51 [sync|nosync] Force or prevent RAID initialization.
53 [rebuild <idx>] Rebuild drive number 'idx' (first drive is 0).
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.
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.
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'.
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 .. .. .. .. .. .. .. .. ..
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'.
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.
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.
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)
135 0 1960893648 raid \
136 raid4 1 2048 \
137 5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81
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
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
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.
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>
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.
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).
213 Discard Support
214 ---------------
215 The implementation of discard support among hardware vendors varies.
216 When a block is discarded, some storage devices will return zeroes when
217 the block is read. These devices set the 'discard_zeroes_data'
218 attribute. Other devices will return random data. Confusingly, some
219 devices that advertise 'discard_zeroes_data' will not reliably return
220 zeroes when discarded blocks are read! Since RAID 4/5/6 uses blocks
221 from a number of devices to calculate parity blocks and (for performance
222 reasons) relies on 'discard_zeroes_data' being reliable, it is important
223 that the devices be consistent. Blocks may be discarded in the middle
224 of a RAID 4/5/6 stripe and if subsequent read results are not
225 consistent, the parity blocks may be calculated differently at any time;
226 making the parity blocks useless for redundancy. It is important to
227 understand how your hardware behaves with discards if you are going to
228 enable discards with RAID 4/5/6.
230 Since the behavior of storage devices is unreliable in this respect,
231 even when reporting 'discard_zeroes_data', by default RAID 4/5/6
232 discard support is disabled -- this ensures data integrity at the
233 expense of losing some performance.
235 Storage devices that properly support 'discard_zeroes_data' are
236 increasingly whitelisted in the kernel and can thus be trusted.
238 For trusted devices, the following dm-raid module parameter can be set
239 to safely enable discard support for RAID 4/5/6:
240 'devices_handle_discards_safely'
243 Version History
244 ---------------
245 1.0.0 Initial version. Support for RAID 4/5/6
246 1.1.0 Added support for RAID 1
247 1.2.0 Handle creation of arrays that contain failed devices.
248 1.3.0 Added support for RAID 10
249 1.3.1 Allow device replacement/rebuild for RAID 10
250 1.3.2 Fix/improve redundancy checking for RAID10
251 1.4.0 Non-functional change. Removes arg from mapping function.
252 1.4.1 RAID10 fix redundancy validation checks (commit 55ebbb5).
253 1.4.2 Add RAID10 "far" and "offset" algorithm support.
254 1.5.0 Add message interface to allow manipulation of the sync_action.
255 New status (STATUSTYPE_INFO) fields: sync_action and mismatch_cnt.
256 1.5.1 Add ability to restore transiently failed devices on resume.
257 1.5.2 'mismatch_cnt' is zero unless [last_]sync_action is "check".
258 1.6.0 Add discard support (and devices_handle_discard_safely module param).
259 1.7.0 Add support for MD RAID0 mappings.