Commit d9bf0b50 authored Jan 13, 2011 by Mikulas Patocka Committed by Alasdair G Kergon Jan 13, 2011

dm io: remove BIO_RW_SYNCIO flag from kcopyd



Remove the REQ_SYNC flag to improve write throughput 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@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

parent 09c9d4c9

drivers/md/dm-kcopyd.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -345,7 +345,7 @@ static int run_io_job(struct kcopyd_job *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,

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