From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=59440 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1OFAvN-00039B-RO
	for qemu-devel@nongnu.org; Thu, 20 May 2010 14:56:54 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69)
	(envelope-from <anthony@codemonkey.ws>) id 1OFAvL-0002iv-Vi
	for qemu-devel@nongnu.org; Thu, 20 May 2010 14:56:53 -0400
Received: from mail-qy0-f173.google.com ([209.85.221.173]:55159)
	by eggs.gnu.org with esmtp (Exim 4.69)
	(envelope-from <anthony@codemonkey.ws>) id 1OFAvL-0002iq-3Z
	for qemu-devel@nongnu.org; Thu, 20 May 2010 14:56:51 -0400
Received: by qyk4 with SMTP id 4so167197qyk.18
	for <qemu-devel@nongnu.org>; Thu, 20 May 2010 11:56:50 -0700 (PDT)
Message-ID: <4BF585EB.7090209@codemonkey.ws>
Date: Thu, 20 May 2010 13:56:43 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] Re: [PATCH] QEMU: Change default disk caching to
	nocache
References: <1274347924-9188-1-git-send-email-Jes.Sorensen@redhat.com>	<1274347924-9188-2-git-send-email-Jes.Sorensen@redhat.com>
	<4BF5542F.1030004@redhat.com>
In-Reply-To: <4BF5542F.1030004@redhat.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
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.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: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: hch@infradead.org, Jes.Sorensen@redhat.com, qemu-devel@nongnu.org, crobinso@redhat.com

On 05/20/2010 10:24 AM, Paolo Bonzini wrote:
> On 05/20/2010 11:32 AM, Jes.Sorensen@redhat.com wrote:
>> +        if (bdrv_flags & BDRV_O_NOCACHE) {
>> +            fprintf(stderr, "qemu: failed to open disk image %s as "
>> +                    "nocache (O_DIRECT) retrying as write-back\n", 
>> file);
>> +            bdrv_flags &= BDRV_O_NOCACHE;
>
> Missing ~ here.
>
>> +            bdrv_flags |= BDRV_O_CACHE_WB;
>> +            if (bdrv_open(dinfo->bdrv, file, bdrv_flags, drv)<  0)
>> +                goto error_open;
>> +        } else {
>
> I think the retry should be done silently if no cache= option is 
> given.  That is cache=none will be the default but:
>
> - if it is not specified and not supported by the image, fall back to 
> writeback with no warning.  However, this is just a QoI issue and can 
> be fixed later.
>
> - if it is specified and not supported by the image, either fall back 
> to writeback with a warning, or fail altogether.  The former would be 
> a change in behavior, so it has to be documented somewhere if it changes.
>
> Or maybe add BDRV_O_CACHE_WT and let the backend decide the default?

It used to be that we had a CACHE_DEFAULT which allowed qcow2 to do 
CACHE_WB by default whereas everything else did CACHE_WT.  The same 
technique could be used to let physical devices do NOCACHE by default.

Regards,

Anthony Liguori

> Paolo
>