From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=42272 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1OuTrg-00031k-Ma
	for qemu-devel@nongnu.org; Sat, 11 Sep 2010 13:27:49 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69)
	(envelope-from <anthony@codemonkey.ws>) id 1OuTrf-0004Aw-Lc
	for qemu-devel@nongnu.org; Sat, 11 Sep 2010 13:27:48 -0400
Received: from mail-yx0-f173.google.com ([209.85.213.173]:59440)
	by eggs.gnu.org with esmtp (Exim 4.69)
	(envelope-from <anthony@codemonkey.ws>) id 1OuTrf-0004Ab-JP
	for qemu-devel@nongnu.org; Sat, 11 Sep 2010 13:27:47 -0400
Received: by yxs7 with SMTP id 7so1824931yxs.4
	for <qemu-devel@nongnu.org>; Sat, 11 Sep 2010 10:27:46 -0700 (PDT)
Message-ID: <4C8BBC0F.2010309@codemonkey.ws>
Date: Sat, 11 Sep 2010 12:27:43 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [PATCH 2/3] block-nbd: fix use of protocols in
	backing files and nbd probing
References: <1284213896-12705-1-git-send-email-aliguori@us.ibm.com>	<1284213896-12705-3-git-send-email-aliguori@us.ibm.com>
	<AANLkTi=AtFRQSUWA=Js1F_JH5agGw=UVYpd2SLZxpZf5@mail.gmail.com>
In-Reply-To: <AANLkTi=AtFRQSUWA=Js1F_JH5agGw=UVYpd2SLZxpZf5@mail.gmail.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: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Juan Quintela <quintela@redhat.com>

On 09/11/2010 11:53 AM, Stefan Hajnoczi wrote:
> On Sat, Sep 11, 2010 at 3:04 PM, Anthony Liguori<aliguori@us.ibm.com>  wrote:
>    
>> Additionally, there's a spurious read when using an nbd protocol that can be
>> quite destructive when using copy-on-read.  Potentially, this can lead to
>> probing an image file over top of NBD but this is completely wrong as NBD
>> devices are not growable.
>>      
> Can you describe the copy-on-read scenario where the 2 KB probe read
> is a problem?
>
> Accessing a fixed size image file over NBD is probably uncommon, but
> I'm not sure if there's a reason to forbid it.
>    

I think the better solution is to explicitly specific raw with nbd.  
IOW, I think -drive file=nbd:localhost:1026,format=raw should work the 
same way.  I still feel slightly weird about probing happening with 
nbd.  It seems like it could only result in badness.

The specific scenario is migration.  I'm using a copy-on-read file on 
the destination and I want to be sure that I don't read any blocks 
(since they're copied) until the source stops execution.

Regards,

Anthony Liguori

> Stefan
>
>