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+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).
+
+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".