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

[Jens Axboe]

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.


--
Jens Axboe

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