[PATCH] optimize one of the cache flushes

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Hi

This patch removes the second cache flush if discard isn't supported. 
The first flush is hard to bypass, so it's not worth doing it.

Mikulas

---

Don't do the second flush if the request isn't supported.

If the request fails with -EOPNOTSUPP, don't perform the second flush.
This can happen with discard+barrier requests. If the device doesn't support
discard, there would be two useless SYNCHRONIZE CACHE commands.

The first dm_flush cannot be so easily optimized out, so we leave it there.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

---
 drivers/md/dm.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6.35-rc3-fast/drivers/md/dm.c
===================================================================
--- linux-2.6.35-rc3-fast.orig/drivers/md/dm.c	2010-07-08 00:11:05.000000000 +0200
+++ linux-2.6.35-rc3-fast/drivers/md/dm.c	2010-07-08 00:12:02.000000000 +0200
@@ -2365,7 +2365,12 @@ static void process_barrier(struct mappe
 
 	if (!bio_empty_barrier(bio)) {
 		__split_and_process_bio(md, bio);
-		dm_flush(md);
+		/*
+		 * If the request isn't supported, don't waste time with
+		 * the second flush.
+		 */
+		if (md->barrier_error != -EOPNOTSUPP)
+			dm_flush(md);
 	}
 
 	if (md->barrier_error != DM_ENDIO_REQUEUE)

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