[dm-devel] [PATCH v2] dm kcopyd: remove SYNC from flags used in dm_io_request

[Mike Snitzer]

From: Mikulas Patocka <mpatocka at redhat.com>

Removing the REQ_SYNC flag improves write throughput twice when writing
to the origin with a snapshot on the same device (using the CFQ I/O
scheduler).

Sequential write throughput (chunksize of 4k, 32k, 512k)
unpatched:  8.5,  8.6,  9.3 MB/s
patched:   15.2, 18.5, 17.5 MB/s

Snapshot exception reallocations are triggered by writes that are
usually async, so mark the associated dm_io_request as async as well.
This helps when using the CFQ I/O scheduler because it has separate
queues for sync and async I/O, async is optimized for throughput, sync
for latency.  With this change we're consciously favoring throughput
over latency.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
 drivers/md/dm-kcopyd.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

v2: updated patch header

Index: linux-2.6.36-rc7-fast/drivers/md/dm-kcopyd.c
===================================================================
--- linux-2.6.36-rc7-fast.orig/drivers/md/dm-kcopyd.c	2010-10-07 16:09:13.000000000 +0200
+++ linux-2.6.36-rc7-fast/drivers/md/dm-kcopyd.c	2010-10-15 03:18:52.000000000 +0200
@@ -345,7 +345,7 @@ static int run_io_job(struct kcopyd_job 
 {
 	int r;
 	struct dm_io_request io_req = {
-		.bi_rw = job->rw | REQ_SYNC | REQ_UNPLUG,
+		.bi_rw = job->rw | REQ_UNPLUG,
 		.mem.type = DM_IO_PAGE_LIST,
 		.mem.ptr.pl = job->pages,
 		.mem.offset = job->offset,