linux-kernel - Re: [PATCH] Fix discard requests accounting in the diskstats

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Date:	Fri, 17 Apr 2009 11:08:52 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	David Woodhouse <david.woodhouse@...el.com>
Cc:	Nikanth Karthikesan <knikanth@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jmarchan@...hat.com" <jmarchan@...hat.com>
Subject: Re: [PATCH] Fix discard requests accounting in the diskstats

On Fri, Apr 17 2009, David Woodhouse wrote:
> On Fri, 2009-04-17 at 08:55 +0100, Nikanth Karthikesan wrote:
> > On Friday 17 April 2009 12:19:12 Jens Axboe wrote:
> > > On Fri, Apr 17 2009, Nikanth Karthikesan wrote:
> > > > When 2 discard requests are merged, the stats gets updated, but we do not
> > > > update statistics normally when a discard request is issued or completed.
> > > > For example the in_flight counter would be decremented when 2 discard
> > > > requests are merged, but it was not at all incremented when they were
> > > > issued, and in_flight counter will not be decremented, when they are
> > > > completed as well.
> > > >
> > > > This patch fixes this by adding discard requests to the statistics.
> > >
> > > This needs rebasing on top of this one:
> > >
> > > http://lkml.org/lkml/2009/4/16/159
> > >
> > > whenever that gets resubmitted as a proper, working patch.
> > >
> > 
> > Ok. I will resend after Jerome resubmits that patch.
> 
> There are more interesting issues with discard requests and elevators
> for now, such as the fact that elevators only actually prevent write
> requests from crossing each other on the queue if the {start,end} range
> precisely matches, not merely if they happen to overlap.
> 
> So all discard requests are submitted as soft barriers for now anyway.

But they are still marked mergeable, so the accounting should be correct
regardless.

-- 
Jens Axboe

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