[PATCH] md: support stripe/offset mode in raid10

The "industry standard" DDF format allows for a stripe/offset layout where
data is duplicated on different stripes.  e.g.

  A  B  C  D
  D  A  B  C
  E  F  G  H
  H  E  F  G

(columns are drives, rows are stripes, LETTERS are chunks of data).

This is similar to raid10's 'far' mode, but not quite the same.  So enhance
'far' mode with a 'far/offset' option which follows the layout of DDFs
stripe/offset.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/include/linux/raid/raid10.h b/include/linux/raid/raid10.h
index b1103298a8c2..c41e56a7c090 100644
--- a/include/linux/raid/raid10.h
+++ b/include/linux/raid/raid10.h
@@ -24,11 +24,16 @@ struct r10_private_data_s {
         int                     far_copies;   /* number of copies layed out
                                                * at large strides across drives
                                                */
+        int                     far_offset;   /* far_copies are offset by 1 stripe
+                                               * instead of many
+                                               */
         int                     copies;       /* near_copies * far_copies.
                                                * must be <= raid_disks
                                                */
         sector_t                stride;       /* distance between far copies.
-                                               * This is size / far_copies
+                                               * This is size / far_copies unless
+                                               * far_offset, in which case it is
+                                               * 1 stripe.
                                                */
 
         int chunk_shift; /* shift from chunks to sectors */