From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:59997)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <kwolf@redhat.com>) id 1bFGN4-0000pQ-1r
	for qemu-devel@nongnu.org; Tue, 21 Jun 2016 03:45:18 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <kwolf@redhat.com>) id 1bFGN2-0002g8-3b
	for qemu-devel@nongnu.org; Tue, 21 Jun 2016 03:45:17 -0400
Date: Tue, 21 Jun 2016 09:45:07 +0200
From: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20160621074507.GA4520@noname.redhat.com>
References: <1466435958-308-1-git-send-email-den@openvz.org>
	<45cce6cb-af87-9e2f-eeb2-4d4f258a3dc9@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <45cce6cb-af87-9e2f-eeb2-4d4f258a3dc9@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 1/1] block: ignore flush requests when
 storage is clean
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel/>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Denis V. Lunev" <den@openvz.org>, qemu-block@nongnu.org, qemu-devel@nongnu.org, Fam Zheng <famz@redhat.com>, Evgeny Yakovlev <eyakovlev@virtuozzo.com>, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

Am 21.06.2016 um 09:32 hat Paolo Bonzini geschrieben:
> 
> 
> On 20/06/2016 17:19, Denis V. Lunev wrote:
> > +    /* Check if storage is actually dirty before flushing to disk */
> > +    if (!bs->dirty) {
> > +        goto flush_parent;
> > +    }
> > +    bs->dirty = false;
> > +
> 
> This should be cleared after the flush is complete.  If you have
> 
>     write begin
>     write end
>     flush #1 begin
>     flush #2 begin
> 
> Then the second flush must only return after the first has finished.

I think clearing bs->dirty after the flush completion wouldn't
necessarily be right either if there are concurrent writes in flight, as
only completed writes are guaranteed to be flushed by it.

Kevin