md: Support write-intent bitmaps with externally managed metadata.

In this case, the metadata needs to not be in the same
sector as the bitmap.
md will not read/write any bitmap metadata.  Config must be
done via sysfs and when a recovery makes the array non-degraded
again, writing 'true' to 'bitmap/can_clear' will allow bits in
the bitmap to be cleared again.

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index fce02073f1a4..d9138885b87f 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -296,6 +296,7 @@ struct mddev_s
                 unsigned long           chunksize;
                 unsigned long           daemon_sleep; /* how many seconds between updates? */
                 unsigned long           max_write_behind; /* write-behind mode */
+                int                     external;
         } bitmap_info;
 
         struct list_head                all_mddevs;