From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:60430) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QF5KC-00016t-Qk for qemu-devel@nongnu.org; Wed, 27 Apr 2011 10:02:46 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QF5K3-0002Zy-FO for qemu-devel@nongnu.org; Wed, 27 Apr 2011 10:02:40 -0400 Received: from fnarfbargle.com ([93.93.131.67]:54133) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QF5K3-0002Zo-9x for qemu-devel@nongnu.org; Wed, 27 Apr 2011 10:02:31 -0400 Message-ID: <4DB821EE.8010706@fnarfbargle.com> Date: Wed, 27 Apr 2011 22:02:22 +0800 From: Brad Campbell MIME-Version: 1.0 References: <4DB7880C.4090302@fnarfbargle.com> <4DB7DA44.4030008@fnarfbargle.com> <4DB7EAAC.3080402@redhat.com> <4DB81E16.5010408@fnarfbargle.com> <4DB8209A.7070400@redhat.com> In-Reply-To: <4DB8209A.7070400@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Qemu-img convert with -B List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Stefan Hajnoczi , qemu-devel@nongnu.org On 27/04/11 21:56, Kevin Wolf wrote: > When you don't have a backing file, leaving an cluster unallocated means > that it's zero. When you have a backing file, it could be anything. So > if qemu-img convert wanted to save this space, it would have to read > from the backing file and leave the cluster unallocated if it reads as zero. > > This is something that qemu-img doesn't do today. ... which would explain the behaviour I'm seeing then. Great, time to dust off my Kernighan & Ritchie I guess. Thanks for the explanation. Regards, Brad