On Thu, 22 Jul 2010, Mikulas Patocka wrote:
> Remove BIO_RW_SYNCIO flag from kcopyd
>
> This improves write throughput twice when writing to the origin with snapshot
> on the same device.
>
> I don't know the exact reason, BIO_RW_SYNCIO is not documented and
> its effects on queueing are not obvious from the code. There seem to
> be two sets of limits, one for sync and the other for async requests.
The reason is that with BIO_RW_SYNCIO CFQ scheduler doesn't merge write
requests. It sends them one-by-one, so if the disk has for example 64
tags, it sends out 64 one-chunk write requests.
On other i/o schedulers (noop and deadline), this patch has no effect.
Mikulas
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>
> ---
> drivers/md/dm-kcopyd.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: linux-2.6.34-fast/drivers/md/dm-kcopyd.c
> ===================================================================
> --- linux-2.6.34-fast.orig/drivers/md/dm-kcopyd.c 2010-07-21 17:57:35.000000000 +0200
> +++ linux-2.6.34-fast/drivers/md/dm-kcopyd.c 2010-07-22 15:24:02.000000000 +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 | (1 << BIO_RW_SYNCIO) | (1 << BIO_RW_UNPLUG),
> + .bi_rw = job->rw | (1 << BIO_RW_UNPLUG),
> .mem.type = DM_IO_PAGE_LIST,
> .mem.ptr.pl = job->pages,
> .mem.offset = job->offset,
>
--
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