dm raid: document RAID 4/5/6 discard support

[linux-2.6-block.git] / Documentation / device-mapper / dm-raid.txt

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>:
  raid1		RAID1 mirroring
  raid4		RAID4 dedicated parity disk
  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
  raid10        Various RAID10 inspired algorithms chosen by additional params
		- 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
  http://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'.

<#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.

	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.

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.
	"reshape"- Currently unsupported (-EINVAL).


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.