<commit_before>Device-mapper RAID (dm-raid) is a bridge from DM to MD.  It
provides a way to use device-mapper interfaces to access the MD RAID
drivers.

As with all device-mapper targets, the nominal public interfaces are the
constructor (CTR) tables and the status outputs (both STATUSTYPE_INFO
and STATUSTYPE_TABLE).  The CTR table looks like the following:

1: <s> <l> raid \
2:      <raid_type> <#raid_params> <raid_params> \
3:      <#raid_devs> <meta_dev1> <dev1> .. <meta_devN> <devN>

Line 1 contains the standard first three arguments to any device-mapper
target - the start, length, and target type fields.  The target type in
this case is "raid".

Line 2 contains the arguments that define the particular raid
type/personality/level, the required arguments for that raid type, and
any optional arguments.  Possible raid types include: raid4, raid5_la,
raid5_ls, raid5_rs, raid6_zr, raid6_nr, and raid6_nc.  (raid1 is
planned for the future.)  The list of required and optional parameters
is the same for all the current raid types.  The required parameters are
positional, while the optional parameters are given as key/value pairs.
The possible parameters are as follows:
 <chunk_size>           Chunk size in sectors.
 [[no]sync]             Force/Prevent RAID initialization
 [rebuild <idx>]        Rebuild the drive indicated by the index
 [daemon_sleep <ms>]    Time between bitmap daemon work to clear bits
 [min_recovery_rate <kB/sec/disk>]      Throttle RAID initialization
 [max_recovery_rate <kB/sec/disk>]      Throttle RAID initialization
 [max_write_behind <sectors>]           See '-write-behind=' (man mdadm)
 [stripe_cache <sectors>]               Stripe cache size for higher RAIDs

Line 3 contains the list of devices that compose the array in
metadata/data device pairs.  If the metadata is stored separately, a '-'
is given for the metadata device position.  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.

NB. Currently all metadata devices must be specified as '-'.

Examples:
# RAID4 - 4 data drives, 1 parity
# 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 (no metadata devices)
# Chunk size of 1MiB, force RAID initialization,
#       min recovery rate at 20 kiB/sec/disk
0 1960893648 raid \
        raid4 4 2048 min_recovery_rate 20 sync\
        5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81

Performing a 'dmsetup table' should display the CTR table used to
construct the mapping (with possible reordering of optional
parameters).

Performing a 'dmsetup status' will yield information on the state and
health of the array.  The output is as follows:
1: <s> <l> raid \
2:      <raid_type> <#devices> <1 health char for each dev> <resync_ratio>

Line 1 is standard DM output.  Line 2 is best shown by example:
        0 1960893648 raid raid4 5 AAAAA 2/490221568
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 recovery.<commit_msg>dm raid: improve table parameters documentation<commit_after>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.

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>:
  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

  Refererence: 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
	[max_write_behind <sectors>]       See '-write-behind=' (man mdadm)
	[stripe_cache <sectors>]           Stripe cache size (higher RAIDs only)

<#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.  Currently, separate metadata devices are not supported and '-'
	is required in place of the metadata device.

	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 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 (no metadata devices)
# Chunk size of 1MiB, force RAID initialization,
#       min recovery rate at 20 kiB/sec/disk

0 1960893648 raid \
        raid4 4 2048 min_recovery_rate 20 sync\
        5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81

'dmsetup table' displays the table used to construct the mapping.
The optional parameters will always be 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.

'dmsetup status' yields information on the state and health of the
array.
The output is as follows:
1: <s> <l> raid \
2:      <raid_type> <#devices> <1 health char for each dev> <resync_ratio>

Line 1 is the standard output produced by device-mapper.
Line 2 is produced by the raid target, and best explained by example:
        0 1960893648 raid raid4 5 AAAAA 2/490221568
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 recovery.
Faulty or missing devices are marked 'D'.  Devices that are out-of-sync
are marked 'a'.
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