Allow raid10 resync to happening in larger chunks.

The raid10 resync/recovery code currently limits the amount of
in-flight resync IO to 2Meg.  This was copied from raid1 where
it seems quite adequate.  However for raid10, some layouts require
a bit of seeking to perform a resync, and allowing a larger buffer
size means that the seeking can be significantly reduced.

There is probably no real need to limit the amount of in-flight
IO at all.  Any shortage of memory will naturally reduce the
amount of buffer space available down to a set minimum, and any
concurrent normal IO will quickly cause resync IO to back off.

The only problem would be that normal IO has to wait for all resync IO
to finish, so a very large amount of resync IO could cause unpleasant
latency when normal IO starts up.

So: increase RESYNC_DEPTH to allow 32Meg of buffer (if memory is
available) which seems to be a good amount.  Also reduce the amount
of memory reserved as there is no need to keep 2Meg just for resync if
memory is tight.

Thanks to Keld for the suggestion.

Cc: Keld Jørn Simonsen <keld@dkuug.dk>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -76,11 +76,13 @@ static void r10bio_pool_free(void *r10_bio, void *data)
 	kfree(r10_bio);
 }
 
+/* Maximum size of each resync request */
 #define RESYNC_BLOCK_SIZE (64*1024)
-//#define RESYNC_BLOCK_SIZE PAGE_SIZE
-#define RESYNC_SECTORS (RESYNC_BLOCK_SIZE >> 9)
 #define RESYNC_PAGES ((RESYNC_BLOCK_SIZE + PAGE_SIZE-1) / PAGE_SIZE)
-#define RESYNC_WINDOW (2048*1024)
+/* amount of memory to reserve for resync requests */
+#define RESYNC_WINDOW (1024*1024)
+/* maximum number of concurrent requests, memory permitting */
+#define RESYNC_DEPTH (32*1024*1024/RESYNC_BLOCK_SIZE)
 
 /*
  * When performing a resync, we need to read and compare, so
@@ -690,7 +692,6 @@ static int flush_pending_writes(conf_t *conf)
  *    there is no normal IO happeing.  It must arrange to call
  *    lower_barrier when the particular background IO completes.
  */
-#define RESYNC_DEPTH 32
 
 static void raise_barrier(conf_t *conf, int force)
 {