424 lines
17 KiB
ReStructuredText
424 lines
17 KiB
ReStructuredText
=======
|
|
dm-raid
|
|
=======
|
|
|
|
The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.
|
|
It allows the MD RAID drivers to be accessed using a device-mapper
|
|
interface.
|
|
|
|
|
|
Mapping Table Interface
|
|
-----------------------
|
|
The target is named "raid" and it accepts the following parameters::
|
|
|
|
<raid_type> <#raid_params> <raid_params> \
|
|
<#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]
|
|
|
|
<raid_type>:
|
|
|
|
============= ===============================================================
|
|
raid0 RAID0 striping (no resilience)
|
|
raid1 RAID1 mirroring
|
|
raid4 RAID4 with dedicated last parity disk
|
|
raid5_n RAID5 with dedicated last parity disk supporting takeover
|
|
Same as raid4
|
|
|
|
- Transitory layout
|
|
raid5_la RAID5 left asymmetric
|
|
|
|
- rotating parity 0 with data continuation
|
|
raid5_ra RAID5 right asymmetric
|
|
|
|
- rotating parity N with data continuation
|
|
raid5_ls RAID5 left symmetric
|
|
|
|
- rotating parity 0 with data restart
|
|
raid5_rs RAID5 right symmetric
|
|
|
|
- rotating parity N with data restart
|
|
raid6_zr RAID6 zero restart
|
|
|
|
- rotating parity zero (left-to-right) with data restart
|
|
raid6_nr RAID6 N restart
|
|
|
|
- rotating parity N (right-to-left) with data restart
|
|
raid6_nc RAID6 N continue
|
|
|
|
- rotating parity N (right-to-left) with data continuation
|
|
raid6_n_6 RAID6 with dedicate parity disks
|
|
|
|
- parity and Q-syndrome on the last 2 disks;
|
|
layout for takeover from/to raid4/raid5_n
|
|
raid6_la_6 Same as "raid_la" plus dedicated last Q-syndrome disk
|
|
|
|
- layout for takeover from raid5_la from/to raid6
|
|
raid6_ra_6 Same as "raid5_ra" dedicated last Q-syndrome disk
|
|
|
|
- layout for takeover from raid5_ra from/to raid6
|
|
raid6_ls_6 Same as "raid5_ls" dedicated last Q-syndrome disk
|
|
|
|
- layout for takeover from raid5_ls from/to raid6
|
|
raid6_rs_6 Same as "raid5_rs" dedicated last Q-syndrome disk
|
|
|
|
- layout for takeover from raid5_rs from/to raid6
|
|
raid10 Various RAID10 inspired algorithms chosen by additional params
|
|
(see raid10_format and raid10_copies below)
|
|
|
|
- RAID10: Striped Mirrors (aka 'Striping on top of mirrors')
|
|
- RAID1E: Integrated Adjacent Stripe Mirroring
|
|
- RAID1E: Integrated Offset Stripe Mirroring
|
|
- and other similar RAID10 variants
|
|
============= ===============================================================
|
|
|
|
Reference: Chapter 4 of
|
|
https://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
|
|
|
|
<#raid_params>: The number of parameters that follow.
|
|
|
|
<raid_params> consists of
|
|
|
|
Mandatory parameters:
|
|
<chunk_size>:
|
|
Chunk size in sectors. This parameter is often known as
|
|
"stripe size". It is the only mandatory parameter and
|
|
is placed first.
|
|
|
|
followed by optional parameters (in any order):
|
|
[sync|nosync]
|
|
Force or prevent RAID initialization.
|
|
|
|
[rebuild <idx>]
|
|
Rebuild drive number 'idx' (first drive is 0).
|
|
|
|
[daemon_sleep <ms>]
|
|
Interval between runs of the bitmap daemon that
|
|
clear bits. A longer interval means less bitmap I/O but
|
|
resyncing after a failure is likely to take longer.
|
|
|
|
[min_recovery_rate <kB/sec/disk>]
|
|
Throttle RAID initialization
|
|
[max_recovery_rate <kB/sec/disk>]
|
|
Throttle RAID initialization
|
|
[write_mostly <idx>]
|
|
Mark drive index 'idx' write-mostly.
|
|
[max_write_behind <sectors>]
|
|
See '--write-behind=' (man mdadm)
|
|
[stripe_cache <sectors>]
|
|
Stripe cache size (RAID 4/5/6 only)
|
|
[region_size <sectors>]
|
|
The region_size multiplied by the number of regions is the
|
|
logical size of the array. The bitmap records the device
|
|
synchronisation state for each region.
|
|
|
|
[raid10_copies <# copies>], [raid10_format <near|far|offset>]
|
|
These two options are used to alter the default layout of
|
|
a RAID10 configuration. The number of copies is can be
|
|
specified, but the default is 2. There are also three
|
|
variations to how the copies are laid down - the default
|
|
is "near". Near copies are what most people think of with
|
|
respect to mirroring. If these options are left unspecified,
|
|
or 'raid10_copies 2' and/or 'raid10_format near' are given,
|
|
then the layouts for 2, 3 and 4 devices are:
|
|
|
|
======== ========== ==============
|
|
2 drives 3 drives 4 drives
|
|
======== ========== ==============
|
|
A1 A1 A1 A1 A2 A1 A1 A2 A2
|
|
A2 A2 A2 A3 A3 A3 A3 A4 A4
|
|
A3 A3 A4 A4 A5 A5 A5 A6 A6
|
|
A4 A4 A5 A6 A6 A7 A7 A8 A8
|
|
.. .. .. .. .. .. .. .. ..
|
|
======== ========== ==============
|
|
|
|
The 2-device layout is equivalent 2-way RAID1. The 4-device
|
|
layout is what a traditional RAID10 would look like. The
|
|
3-device layout is what might be called a 'RAID1E - Integrated
|
|
Adjacent Stripe Mirroring'.
|
|
|
|
If 'raid10_copies 2' and 'raid10_format far', then the layouts
|
|
for 2, 3 and 4 devices are:
|
|
|
|
======== ============ ===================
|
|
2 drives 3 drives 4 drives
|
|
======== ============ ===================
|
|
A1 A2 A1 A2 A3 A1 A2 A3 A4
|
|
A3 A4 A4 A5 A6 A5 A6 A7 A8
|
|
A5 A6 A7 A8 A9 A9 A10 A11 A12
|
|
.. .. .. .. .. .. .. .. ..
|
|
A2 A1 A3 A1 A2 A2 A1 A4 A3
|
|
A4 A3 A6 A4 A5 A6 A5 A8 A7
|
|
A6 A5 A9 A7 A8 A10 A9 A12 A11
|
|
.. .. .. .. .. .. .. .. ..
|
|
======== ============ ===================
|
|
|
|
If 'raid10_copies 2' and 'raid10_format offset', then the
|
|
layouts for 2, 3 and 4 devices are:
|
|
|
|
======== ========== ================
|
|
2 drives 3 drives 4 drives
|
|
======== ========== ================
|
|
A1 A2 A1 A2 A3 A1 A2 A3 A4
|
|
A2 A1 A3 A1 A2 A2 A1 A4 A3
|
|
A3 A4 A4 A5 A6 A5 A6 A7 A8
|
|
A4 A3 A6 A4 A5 A6 A5 A8 A7
|
|
A5 A6 A7 A8 A9 A9 A10 A11 A12
|
|
A6 A5 A9 A7 A8 A10 A9 A12 A11
|
|
.. .. .. .. .. .. .. .. ..
|
|
======== ========== ================
|
|
|
|
Here we see layouts closely akin to 'RAID1E - Integrated
|
|
Offset Stripe Mirroring'.
|
|
|
|
[delta_disks <N>]
|
|
The delta_disks option value (-251 < N < +251) triggers
|
|
device removal (negative value) or device addition (positive
|
|
value) to any reshape supporting raid levels 4/5/6 and 10.
|
|
RAID levels 4/5/6 allow for addition of devices (metadata
|
|
and data device tuple), raid10_near and raid10_offset only
|
|
allow for device addition. raid10_far does not support any
|
|
reshaping at all.
|
|
A minimum of devices have to be kept to enforce resilience,
|
|
which is 3 devices for raid4/5 and 4 devices for raid6.
|
|
|
|
[data_offset <sectors>]
|
|
This option value defines the offset into each data device
|
|
where the data starts. This is used to provide out-of-place
|
|
reshaping space to avoid writing over data while
|
|
changing the layout of stripes, hence an interruption/crash
|
|
may happen at any time without the risk of losing data.
|
|
E.g. when adding devices to an existing raid set during
|
|
forward reshaping, the out-of-place space will be allocated
|
|
at the beginning of each raid device. The kernel raid4/5/6/10
|
|
MD personalities supporting such device addition will read the data from
|
|
the existing first stripes (those with smaller number of stripes)
|
|
starting at data_offset to fill up a new stripe with the larger
|
|
number of stripes, calculate the redundancy blocks (CRC/Q-syndrome)
|
|
and write that new stripe to offset 0. Same will be applied to all
|
|
N-1 other new stripes. This out-of-place scheme is used to change
|
|
the RAID type (i.e. the allocation algorithm) as well, e.g.
|
|
changing from raid5_ls to raid5_n.
|
|
|
|
[journal_dev <dev>]
|
|
This option adds a journal device to raid4/5/6 raid sets and
|
|
uses it to close the 'write hole' caused by the non-atomic updates
|
|
to the component devices which can cause data loss during recovery.
|
|
The journal device is used as writethrough thus causing writes to
|
|
be throttled versus non-journaled raid4/5/6 sets.
|
|
Takeover/reshape is not possible with a raid4/5/6 journal device;
|
|
it has to be deconfigured before requesting these.
|
|
|
|
[journal_mode <mode>]
|
|
This option sets the caching mode on journaled raid4/5/6 raid sets
|
|
(see 'journal_dev <dev>' above) to 'writethrough' or 'writeback'.
|
|
If 'writeback' is selected the journal device has to be resilient
|
|
and must not suffer from the 'write hole' problem itself (e.g. use
|
|
raid1 or raid10) to avoid a single point of failure.
|
|
|
|
<#raid_devs>: The number of devices composing the array.
|
|
Each device consists of two entries. The first is the device
|
|
containing the metadata (if any); the second is the one containing the
|
|
data. A Maximum of 64 metadata/data device entries are supported
|
|
up to target version 1.8.0.
|
|
1.9.0 supports up to 253 which is enforced by the used MD kernel runtime.
|
|
|
|
If a drive has failed or is missing at creation time, a '-' can be
|
|
given for both the metadata and data drives for a given position.
|
|
|
|
|
|
Example Tables
|
|
--------------
|
|
|
|
::
|
|
|
|
# RAID4 - 4 data drives, 1 parity (no metadata devices)
|
|
# No metadata devices specified to hold superblock/bitmap info
|
|
# Chunk size of 1MiB
|
|
# (Lines separated for easy reading)
|
|
|
|
0 1960893648 raid \
|
|
raid4 1 2048 \
|
|
5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81
|
|
|
|
# RAID4 - 4 data drives, 1 parity (with metadata devices)
|
|
# Chunk size of 1MiB, force RAID initialization,
|
|
# min recovery rate at 20 kiB/sec/disk
|
|
|
|
0 1960893648 raid \
|
|
raid4 4 2048 sync min_recovery_rate 20 \
|
|
5 8:17 8:18 8:33 8:34 8:49 8:50 8:65 8:66 8:81 8:82
|
|
|
|
|
|
Status Output
|
|
-------------
|
|
'dmsetup table' displays the table used to construct the mapping.
|
|
The optional parameters are always printed in the order listed
|
|
above with "sync" or "nosync" always output ahead of the other
|
|
arguments, regardless of the order used when originally loading the table.
|
|
Arguments that can be repeated are ordered by value.
|
|
|
|
|
|
'dmsetup status' yields information on the state and health of the array.
|
|
The output is as follows (normally a single line, but expanded here for
|
|
clarity)::
|
|
|
|
1: <s> <l> raid \
|
|
2: <raid_type> <#devices> <health_chars> \
|
|
3: <sync_ratio> <sync_action> <mismatch_cnt>
|
|
|
|
Line 1 is the standard output produced by device-mapper.
|
|
|
|
Line 2 & 3 are produced by the raid target and are best explained by example::
|
|
|
|
0 1960893648 raid raid4 5 AAAAA 2/490221568 init 0
|
|
|
|
Here we can see the RAID type is raid4, there are 5 devices - all of
|
|
which are 'A'live, and the array is 2/490221568 complete with its initial
|
|
recovery. Here is a fuller description of the individual fields:
|
|
|
|
=============== =========================================================
|
|
<raid_type> Same as the <raid_type> used to create the array.
|
|
<health_chars> One char for each device, indicating:
|
|
|
|
- 'A' = alive and in-sync
|
|
- 'a' = alive but not in-sync
|
|
- 'D' = dead/failed.
|
|
<sync_ratio> The ratio indicating how much of the array has undergone
|
|
the process described by 'sync_action'. If the
|
|
'sync_action' is "check" or "repair", then the process
|
|
of "resync" or "recover" can be considered complete.
|
|
<sync_action> One of the following possible states:
|
|
|
|
idle
|
|
- No synchronization action is being performed.
|
|
frozen
|
|
- The current action has been halted.
|
|
resync
|
|
- Array is undergoing its initial synchronization
|
|
or is resynchronizing after an unclean shutdown
|
|
(possibly aided by a bitmap).
|
|
recover
|
|
- A device in the array is being rebuilt or
|
|
replaced.
|
|
check
|
|
- A user-initiated full check of the array is
|
|
being performed. All blocks are read and
|
|
checked for consistency. The number of
|
|
discrepancies found are recorded in
|
|
<mismatch_cnt>. No changes are made to the
|
|
array by this action.
|
|
repair
|
|
- The same as "check", but discrepancies are
|
|
corrected.
|
|
reshape
|
|
- The array is undergoing a reshape.
|
|
<mismatch_cnt> The number of discrepancies found between mirror copies
|
|
in RAID1/10 or wrong parity values found in RAID4/5/6.
|
|
This value is valid only after a "check" of the array
|
|
is performed. A healthy array has a 'mismatch_cnt' of 0.
|
|
<data_offset> The current data offset to the start of the user data on
|
|
each component device of a raid set (see the respective
|
|
raid parameter to support out-of-place reshaping).
|
|
<journal_char> - 'A' - active write-through journal device.
|
|
- 'a' - active write-back journal device.
|
|
- 'D' - dead journal device.
|
|
- '-' - no journal device.
|
|
=============== =========================================================
|
|
|
|
|
|
Message Interface
|
|
-----------------
|
|
The dm-raid target will accept certain actions through the 'message' interface.
|
|
('man dmsetup' for more information on the message interface.) These actions
|
|
include:
|
|
|
|
========= ================================================
|
|
"idle" Halt the current sync action.
|
|
"frozen" Freeze the current sync action.
|
|
"resync" Initiate/continue a resync.
|
|
"recover" Initiate/continue a recover process.
|
|
"check" Initiate a check (i.e. a "scrub") of the array.
|
|
"repair" Initiate a repair of the array.
|
|
========= ================================================
|
|
|
|
|
|
Discard Support
|
|
---------------
|
|
The implementation of discard support among hardware vendors varies.
|
|
When a block is discarded, some storage devices will return zeroes when
|
|
the block is read. These devices set the 'discard_zeroes_data'
|
|
attribute. Other devices will return random data. Confusingly, some
|
|
devices that advertise 'discard_zeroes_data' will not reliably return
|
|
zeroes when discarded blocks are read! Since RAID 4/5/6 uses blocks
|
|
from a number of devices to calculate parity blocks and (for performance
|
|
reasons) relies on 'discard_zeroes_data' being reliable, it is important
|
|
that the devices be consistent. Blocks may be discarded in the middle
|
|
of a RAID 4/5/6 stripe and if subsequent read results are not
|
|
consistent, the parity blocks may be calculated differently at any time;
|
|
making the parity blocks useless for redundancy. It is important to
|
|
understand how your hardware behaves with discards if you are going to
|
|
enable discards with RAID 4/5/6.
|
|
|
|
Since the behavior of storage devices is unreliable in this respect,
|
|
even when reporting 'discard_zeroes_data', by default RAID 4/5/6
|
|
discard support is disabled -- this ensures data integrity at the
|
|
expense of losing some performance.
|
|
|
|
Storage devices that properly support 'discard_zeroes_data' are
|
|
increasingly whitelisted in the kernel and can thus be trusted.
|
|
|
|
For trusted devices, the following dm-raid module parameter can be set
|
|
to safely enable discard support for RAID 4/5/6:
|
|
|
|
'devices_handle_discards_safely'
|
|
|
|
|
|
Version History
|
|
---------------
|
|
|
|
::
|
|
|
|
1.0.0 Initial version. Support for RAID 4/5/6
|
|
1.1.0 Added support for RAID 1
|
|
1.2.0 Handle creation of arrays that contain failed devices.
|
|
1.3.0 Added support for RAID 10
|
|
1.3.1 Allow device replacement/rebuild for RAID 10
|
|
1.3.2 Fix/improve redundancy checking for RAID10
|
|
1.4.0 Non-functional change. Removes arg from mapping function.
|
|
1.4.1 RAID10 fix redundancy validation checks (commit 55ebbb5).
|
|
1.4.2 Add RAID10 "far" and "offset" algorithm support.
|
|
1.5.0 Add message interface to allow manipulation of the sync_action.
|
|
New status (STATUSTYPE_INFO) fields: sync_action and mismatch_cnt.
|
|
1.5.1 Add ability to restore transiently failed devices on resume.
|
|
1.5.2 'mismatch_cnt' is zero unless [last_]sync_action is "check".
|
|
1.6.0 Add discard support (and devices_handle_discard_safely module param).
|
|
1.7.0 Add support for MD RAID0 mappings.
|
|
1.8.0 Explicitly check for compatible flags in the superblock metadata
|
|
and reject to start the raid set if any are set by a newer
|
|
target version, thus avoiding data corruption on a raid set
|
|
with a reshape in progress.
|
|
1.9.0 Add support for RAID level takeover/reshape/region size
|
|
and set size reduction.
|
|
1.9.1 Fix activation of existing RAID 4/10 mapped devices
|
|
1.9.2 Don't emit '- -' on the status table line in case the constructor
|
|
fails reading a superblock. Correctly emit 'maj:min1 maj:min2' and
|
|
'D' on the status line. If '- -' is passed into the constructor, emit
|
|
'- -' on the table line and '-' as the status line health character.
|
|
1.10.0 Add support for raid4/5/6 journal device
|
|
1.10.1 Fix data corruption on reshape request
|
|
1.11.0 Fix table line argument order
|
|
(wrong raid10_copies/raid10_format sequence)
|
|
1.11.1 Add raid4/5/6 journal write-back support via journal_mode option
|
|
1.12.1 Fix for MD deadlock between mddev_suspend() and md_write_start() available
|
|
1.13.0 Fix dev_health status at end of "recover" (was 'a', now 'A')
|
|
1.13.1 Fix deadlock caused by early md_stop_writes(). Also fix size an
|
|
state races.
|
|
1.13.2 Fix raid redundancy validation and avoid keeping raid set frozen
|
|
1.14.0 Fix reshape race on small devices. Fix stripe adding reshape
|
|
deadlock/potential data corruption. Update superblock when
|
|
specific devices are requested via rebuild. Fix RAID leg
|
|
rebuild errors.
|
|
1.15.0 Fix size extensions not being synchronized in case of new MD bitmap
|
|
pages allocated; also fix those not occurring after previous reductions
|
|
1.15.1 Fix argument count and arguments for rebuild/write_mostly/journal_(dev|mode)
|
|
on the status line.
|