Re: [PATCH RFC] blkcg: prepare blkcg knobs for default hierarchy

[Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, Apr 23, 2014 at 03:00:43PM -0400, Tejun Heo wrote:
> Hello,
> 
> On Wed, Apr 23, 2014 at 02:58:35PM -0400, Vivek Goyal wrote:
> > Oh, sorry, I had misunderstood your question.
> > 
> > - Number of IOs serviced will be different at throttling layer and
> >   CFQ layer as throttling accounts IO in terms of bios and CFQ
> >   accounts in terms of number of requests.
> 
> But shouldn't it be possible to determine that from merged count?  Or
> we can just expose both bio and request counts at block core layer.
> The point is that there's nothing controller specific about these
> stats.

In general this idea makes sense. Exporting both request and bio will
solve the problem of io accounting. Also that should allow us to
get rid of blkio.io_merged.

What about sync/async differentiation? Throttling layer seems to flag a request sync
only if bio->bi_rw flag has REQ_SYNC set. While CFQ seems to consider
request sync if bio is either read or bio->bi_rw has REQ_SYNC flag set.

So we need to make this definition uniform. Or I am wondering do we
really need to export sync/async data. (Again put in by google folks).
How useful this info really is. 

Thanks
Vivek