Date: Sat, 21 Mar 2009 03:03:09 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Jerome Marchand <jmarchan@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] block: forbid to re-enable I/O stat accounting
Message-Id: <20090321030309.f783ec8c.akpm@linux-foundation.org>
In-Reply-To: <200903201333.n2KDXONh023780@dhcp-0-152.brq.redhat.com>
References: <200903201333.n2KDXONh023780@dhcp-0-152.brq.redhat.com>
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Please fix your email headers - what I received was

From: Jerome Marchand <jmarchan@redhat.com>
To: unlisted-recipients:;;@imap1.linux-foundation.org (no To-header on input)
Cc: Jens Axboe <axboe@kernel.dk>


On Thu, 19 Mar 2009 11:36:50 +0100 Jerome Marchand <jmarchan@redhat.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?

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