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Linux kernel mirror (for testing)
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kernel
os
linux
1dm-raid
2-------
3
4The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.
5It allows the MD RAID drivers to be accessed using a device-mapper
6interface.
7
8The target is named "raid" and it accepts the following parameters:
9
10 <raid_type> <#raid_params> <raid_params> \
11 <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]
12
13<raid_type>:
14 raid1 RAID1 mirroring
15 raid4 RAID4 dedicated parity disk
16 raid5_la RAID5 left asymmetric
17 - rotating parity 0 with data continuation
18 raid5_ra RAID5 right asymmetric
19 - rotating parity N with data continuation
20 raid5_ls RAID5 left symmetric
21 - rotating parity 0 with data restart
22 raid5_rs RAID5 right symmetric
23 - rotating parity N with data restart
24 raid6_zr RAID6 zero restart
25 - rotating parity zero (left-to-right) with data restart
26 raid6_nr RAID6 N restart
27 - rotating parity N (right-to-left) with data restart
28 raid6_nc RAID6 N continue
29 - rotating parity N (right-to-left) with data continuation
30 raid10 Various RAID10 inspired algorithms chosen by additional params
31 - RAID10: Striped Mirrors (aka 'Striping on top of mirrors')
32 - RAID1E: Integrated Adjacent Stripe Mirroring
33 - and other similar RAID10 variants
34
35 Reference: Chapter 4 of
36 http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
37
38<#raid_params>: The number of parameters that follow.
39
40<raid_params> consists of
41 Mandatory parameters:
42 <chunk_size>: Chunk size in sectors. This parameter is often known as
43 "stripe size". It is the only mandatory parameter and
44 is placed first.
45
46 followed by optional parameters (in any order):
47 [sync|nosync] Force or prevent RAID initialization.
48
49 [rebuild <idx>] Rebuild drive number idx (first drive is 0).
50
51 [daemon_sleep <ms>]
52 Interval between runs of the bitmap daemon that
53 clear bits. A longer interval means less bitmap I/O but
54 resyncing after a failure is likely to take longer.
55
56 [min_recovery_rate <kB/sec/disk>] Throttle RAID initialization
57 [max_recovery_rate <kB/sec/disk>] Throttle RAID initialization
58 [write_mostly <idx>] Drive index is write-mostly
59 [max_write_behind <sectors>] See '-write-behind=' (man mdadm)
60 [stripe_cache <sectors>] Stripe cache size (higher RAIDs only)
61 [region_size <sectors>]
62 The region_size multiplied by the number of regions is the
63 logical size of the array. The bitmap records the device
64 synchronisation state for each region.
65
66 [raid10_copies <# copies>]
67 [raid10_format near]
68 These two options are used to alter the default layout of
69 a RAID10 configuration. The number of copies is can be
70 specified, but the default is 2. There are other variations
71 to how the copies are laid down - the default and only current
72 option is "near". Near copies are what most people think of
73 with respect to mirroring. If these options are left
74 unspecified, or 'raid10_copies 2' and/or 'raid10_format near'
75 are given, then the layouts for 2, 3 and 4 devices are:
76 2 drives 3 drives 4 drives
77 -------- ---------- --------------
78 A1 A1 A1 A1 A2 A1 A1 A2 A2
79 A2 A2 A2 A3 A3 A3 A3 A4 A4
80 A3 A3 A4 A4 A5 A5 A5 A6 A6
81 A4 A4 A5 A6 A6 A7 A7 A8 A8
82 .. .. .. .. .. .. .. .. ..
83 The 2-device layout is equivalent 2-way RAID1. The 4-device
84 layout is what a traditional RAID10 would look like. The
85 3-device layout is what might be called a 'RAID1E - Integrated
86 Adjacent Stripe Mirroring'.
87
88<#raid_devs>: The number of devices composing the array.
89 Each device consists of two entries. The first is the device
90 containing the metadata (if any); the second is the one containing the
91 data.
92
93 If a drive has failed or is missing at creation time, a '-' can be
94 given for both the metadata and data drives for a given position.
95
96
97Example tables
98--------------
99# RAID4 - 4 data drives, 1 parity (no metadata devices)
100# No metadata devices specified to hold superblock/bitmap info
101# Chunk size of 1MiB
102# (Lines separated for easy reading)
103
1040 1960893648 raid \
105 raid4 1 2048 \
106 5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81
107
108# RAID4 - 4 data drives, 1 parity (with metadata devices)
109# Chunk size of 1MiB, force RAID initialization,
110# min recovery rate at 20 kiB/sec/disk
111
1120 1960893648 raid \
113 raid4 4 2048 sync min_recovery_rate 20 \
114 5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82
115
116'dmsetup table' displays the table used to construct the mapping.
117The optional parameters are always printed in the order listed
118above with "sync" or "nosync" always output ahead of the other
119arguments, regardless of the order used when originally loading the table.
120Arguments that can be repeated are ordered by value.
121
122'dmsetup status' yields information on the state and health of the
123array.
124The output is as follows:
1251: <s> <l> raid \
1262: <raid_type> <#devices> <1 health char for each dev> <resync_ratio>
127
128Line 1 is the standard output produced by device-mapper.
129Line 2 is produced by the raid target, and best explained by example:
130 0 1960893648 raid raid4 5 AAAAA 2/490221568
131Here we can see the RAID type is raid4, there are 5 devices - all of
132which are 'A'live, and the array is 2/490221568 complete with recovery.
133Faulty or missing devices are marked 'D'. Devices that are out-of-sync
134are marked 'a'.