From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MlWZ7-0001sI-Fa for qemu-devel@nongnu.org; Wed, 09 Sep 2009 19:27:05 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MlWZ1-0001nm-1p for qemu-devel@nongnu.org; Wed, 09 Sep 2009 19:27:03 -0400 Received: from [199.232.76.173] (port=59179 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MlWYz-0001mL-KE for qemu-devel@nongnu.org; Wed, 09 Sep 2009 19:26:57 -0400 Received: from mail2.shareable.org ([80.68.89.115]:56742) by monty-python.gnu.org with esmtps (TLS-1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.60) (envelope-from ) id 1MlWYy-00029W-BC for qemu-devel@nongnu.org; Wed, 09 Sep 2009 19:26:56 -0400 Received: from jamie by mail2.shareable.org with local (Exim 4.63) (envelope-from ) id 1MlWYw-0005WE-84 for qemu-devel@nongnu.org; Thu, 10 Sep 2009 00:26:54 +0100 Date: Thu, 10 Sep 2009 00:26:54 +0100 From: Jamie Lokier Message-ID: <20090909232654.GA20719@shareable.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Subject: [Qemu-devel] More qcow2 bugs? - qemu-img convert/commit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Today I decided to take a gamble and run "qemu-img commit" on a couple of qcow2 files that have backing files, because the qcow2s had become as large as their backing file anyway. The first one I did without thinking, confident that qcow2 bugs have been squashed. Foolish me, I put my faith in the tool and didn't keep a backup of the VM disk images. (They are quite large after all). Then I got nervous, and for the second one I did this: qemu-img convert -O raw my_qcow2_branch flattened qemu-img commit my_qcow2_branch md5sum my_qcow2_backing_file flattened The two md5sums _should_ be identical, shouldn't they? In mine, the files have the same size but different md5sums. The tool I used is qemu-img from kvm-88, which is the most recent release on kvm's home page. I ran script to find which sectors differed, and found a large number. I stopped counting after the first million sectors, which is a lot, considering the raw image is only 11GB. Extrapolating, about 37% of all the sectors did not match. In all cases, the output of "convert" had all-zeros in those sectors while the backing file after "commit" had non-zero data. To which I think *phew*. At least that gives me some confidence that I didn't trash the file I ran "commit" on, and thank goodness I didn't decide to use "convert" instead. I had in the back of my mind that "convert" might be safer because it would use more tested code paths than "commit" - after all it could just use the same code as qemu and qemu-nbd, whereas "commit" is more special. 37% is large enough to be a clue: Does it simply expand the qcow2 file without reading the backing file at all? A quick strace shows that indeed, it does *open* the backing file and it does read the backing file's header, but after that it doesn't read it at all. Did I miss something obvious which means this is ok? Should the output of "qemu-img convert" be identical to the backing file after "qemu-img commit", assuming they are the same format? Thanks, -- Jamie