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

[Jens Axboe]

On 05/13/2015 11:22 AM, Shaohua Li wrote:
> 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@xxxxxx> 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@xxxxxx>
> > > > ---
> > > >   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?

We can't break then, I was just curious if adding a field to the end of 
the diskstats would potentially not break old applications. If not, they 
could just be updated to grab the new field too.

--
Jens Axboe

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