DM RAID: Add support for MD's RAID10 "far" and "offset" algorithms

DM RAID:  Add support for MD's RAID10 "far" and "offset" algorithms

Until now, dm-raid.c only supported the "near" algorthm of MD's RAID10
implementation.  This patch adds support for the "far" and "offset"
algorithms, but only with the improved redundancy that is brought with
the introduction of the 'use_far_sets' bit, which shifts copied stripes
according to smaller sets vs the entire array.  That is, the 17th bit
of the 'layout' variable that defines the RAID10 implementation will
always be set.   (More information on how the 'layout' variable selects
the RAID10 algorithm can be found in the opening comments of
drivers/md/raid10.c.)

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 37 insertions, 7 deletions

diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt
index 56fb62b09fc5..b428556197c9 100644
--- a/Documentation/device-mapper/dm-raid.txt
+++ b/Documentation/device-mapper/dm-raid.txt
@@ -30,6 +30,7 @@ The target is named "raid" and it accepts the following parameters:
   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
@@ -64,15 +65,15 @@ The target is named "raid" and it accepts the following parameters:
                 synchronisation state for each region.
 
         [raid10_copies   <# copies>]
-        [raid10_format   near]
+        [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 other variations
-                to how the copies are laid down - the default and only current
-                option 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:
+                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
@@ -85,6 +86,33 @@ The target is named "raid" and it accepts the following parameters:
                 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
@@ -142,3 +170,5 @@ Version History
 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   Add RAID10 "far" and "offset" algorithm support.