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[dm-devel] [PATCH] optimize one of the cache flushes


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.



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 redhat com>

 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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