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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>
This commit is contained in:
Mikulas Patocka 2011-01-13 19:59:47 +00:00 committed by Alasdair G Kergon
parent 09c9d4c9b6
commit d9bf0b508d
1 changed files with 1 additions and 1 deletions
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2
drivers/md/dm-kcopyd.c
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@ -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,