linux-kernel - Re: [PATCH] block: forbid to re-enable I/O stat accounting

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Message-ID: <20090321111400.GJ27476@kernel.dk>
Date:	Sat, 21 Mar 2009 12:14:00 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jerome Marchand <jmarchan@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: forbid to re-enable I/O stat accounting

On Sat, Mar 21 2009, Andrew Morton wrote:
> 
> Please fix your email headers - what I received was
> 
> From: Jerome Marchand <jmarchan@...hat.com>
> To: unlisted-recipients:;;@imap1.linux-foundation.org (no To-header on input)
> Cc: Jens Axboe <axboe@...nel.dk>

Ditto, I didn't think anyone else got it. At least noone else got my
reply :-)

> On Thu, 19 Mar 2009 11:36:50 +0100 Jerome Marchand <jmarchan@...hat.com> wrote:
> 
> > When we stop I/O stat accounting we stop to update the in-flight
> > requests counter and we need this counter to be reliable for
> > accounting I/O stats. Unfortunately updating in_flight field may
> > affect performance. So, until we have a better solution, just forbid
> > to re-enable I/O stat accounting after it has been disabled.
> 
> hm.  Is it really so hard to just quiesce the device until all in-flight
> requests have drained?  freeze_bdev() might be a suitable starting point?

That was my suggestion as well, we really need to be able to turn it
back on again. Ideally, I'd like iostat and friends to turn it on/off
when somebody does the monitoring. A one-way street option is not very
nice.

-- 
Jens Axboe

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