[PATCH] md: Only checkpoint expansion progress occasionally

Instead of checkpointing at each stripe, only checkpoint when a new write
would overwrite uncheckpointed data.  Block any write to the uncheckpointed
area.  Arbitrarily checkpoint at least every 3Meg.

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

include/linux/raid/raid5.h

@@ -217,6 +217,9 @@ struct raid5_private_data {
 
 	/* used during an expand */
 	sector_t		expand_progress;	/* MaxSector when no expand happening */
+	sector_t		expand_lo; /* from here up to expand_progress it out-of-bounds
+					    * as we haven't flushed the metadata yet
+					    */
 	int			previous_raid_disks;
 
 	struct list_head	handle_list; /* stripes needing handling */