From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1JfJvr-0002TB-Qq
	for qemu-devel@nongnu.org; Fri, 28 Mar 2008 15:08:07 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1JfJvq-0002Sk-9U
	for qemu-devel@nongnu.org; Fri, 28 Mar 2008 15:08:07 -0400
Received: from [199.232.76.173] (helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1JfJvq-0002Sh-4y
	for qemu-devel@nongnu.org; Fri, 28 Mar 2008 15:08:06 -0400
Received: from kanga.kvack.org ([66.96.29.28])
	by monty-python.gnu.org with esmtp (Exim 4.60)
	(envelope-from <marcelo@kvack.org>) id 1JfJvq-0004Lr-0L
	for qemu-devel@nongnu.org; Fri, 28 Mar 2008 15:08:06 -0400
Date: Fri, 28 Mar 2008 16:11:08 -0300
From: Marcelo Tosatti <marcelo@kvack.org>
Subject: Re: [kvm-devel] [Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush
	request
Message-ID: <20080328191108.GC19547@dmt>
References: <20080328150517.GA18077@dmt>
	<18413.10773.769938.658683@mariner.uk.xensource.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <18413.10773.769938.658683@mariner.uk.xensource.com>
Reply-To: qemu-devel@nongnu.org
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: qemu-devel@nongnu.org

On Fri, Mar 28, 2008 at 05:25:41PM +0000, Ian Jackson wrote:
> Marcelo Tosatti writes ("[Qemu-devel] [PATCH] QEMU: fsync AIO writes on flush request "):
> > Its necessary to guarantee that pending AIO writes have reached stable
> > storage when the flush request returns.
> 
> Surely it isn't necessary to call qemu_aio_flush ?  Because those
> pending AIO writes have not yet been returned to the guest as
> complete, the guest is not entitled to assume that a FLUSH CACHE
> command (issued before those writes have completed) completing
> successfully means that those interleaved writes have reached stable
> storage.

By the time qemu_aio_flush() finishes, all pending requests submitted
before aio_fsync() will have reached stable storage (aio_fsync will be
queued in the last position of the priority 0 AIO queue).

Unless the scheduling priority of the QEMU process is changed between
AIO write's and the fsync().

> Also, this patch does a synchronous flush (which is bad because it
> stalls the guest while the flush takes place) and it ignores any error
> return (which is quite bad - see my other messages about bdrv_flush,
> caches, etc.)
> 
> So I think it would be better to apply
>   - my bdrv_flush patch from February which I've reposted today
> and then
>   - the asynchronous FLUSH CACHE patch which I've posted today
> 
> I think we concluded last time that the change of fsync to fdatasync
> is correct but I think we should wait for the dust to settle before
> introducing another change on top of all this ...

Alright, please also fix the SCSI driver to perform asynchronous flush
with bdrv_aio_flush() and complete the SYNC_CACHE command on the callback.