Re: [PATCH] blk: don't account discard request size

[Shaohua Li <shli@fb.com>]

On Wed, May 13, 2015 at 10:20:12AM -0400, Jens Axboe wrote:
> On 05/13/2015 09:10 AM, Jeff Moyer wrote:
> >Shaohua Li <shli@fb.com> writes:
> >
> >>In a workload with discard request, the IO throughput is generally much
> >>higher than expected. This is quite confusing checking iostat. Discard
> >>request doesn't really write data to drive, so don't account it.
> >>
> >>Signed-off-by: Shaohua Li <shli@fb.com>
> >>---
> >>  block/blk-core.c | 6 +++++-
> >>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>
> >>diff --git a/block/blk-core.c b/block/blk-core.c
> >>index fd154b9..0128d18 100644
> >>--- a/block/blk-core.c
> >>+++ b/block/blk-core.c
> >>@@ -2138,7 +2138,11 @@ EXPORT_SYMBOL_GPL(blk_rq_err_bytes);
> >>
> >>  void blk_account_io_completion(struct request *req, unsigned int bytes)
> >>  {
> >>-	if (blk_do_io_stat(req)) {
> >>+	/*
> >>+	 * discard request doesn't really write @bytes to drive,
> >>+	 * doesn't account it
> >>+	 **/
> >>+	if (blk_do_io_stat(req) && !(req->cmd_flags & REQ_DISCARD)) {
> >>  		const int rw = rq_data_dir(req);
> >>  		struct hd_struct *part;
> >>  		int cpu;
> >
> >I think you want to modify __get_request to not set REQ_IO_STAT for
> >discard requests.  This patch will still account the start of I/O, which
> >means in_flight will be off.
> 
> That would be better. But I'm still not sure we want to turn off
> accounting for discards. For the mixed write/discard cases it's
> definitely confusing. The better option would be to account it as a
> discard and not a write. Preferably in a way that would not break
> existing tools, but so that they could get updated to support it.

It's intentional discard IO start gets accounted, so tools will show
there is IO. I'm not sure if this is better though.

Adding separate columns for discard (maybe flush too) is definitely
preferred. Is breaking existing tools really ok?

Thanks,
Shaohua