From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33062) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XdJ7l-0001fu-V6 for qemu-devel@nongnu.org; Sun, 12 Oct 2014 09:23:56 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XdJ7f-00073w-PZ for qemu-devel@nongnu.org; Sun, 12 Oct 2014 09:23:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:17464) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XdJ7f-00073s-IN for qemu-devel@nongnu.org; Sun, 12 Oct 2014 09:23:43 -0400 Message-ID: <543A80DA.4090201@redhat.com> Date: Sun, 12 Oct 2014 15:23:38 +0200 From: Max Reitz MIME-Version: 1.0 References: <201410091917519618804@sangfor.com> <201410100954567266628@sangfor.com> In-Reply-To: <201410100954567266628@sangfor.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [question] is it possible that big-endian l1 table offset referenced by other I/O while updating l1 table offset in qcow2_update_snapshot_refcount? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Zhang Haoyu , Eric Blake , qemu-devel Am 10.10.2014 um 03:54 schrieb Zhang Haoyu: >>> Hi, >>> I encounter a problem that after deleting snapshot, the qcow2 image size is very larger than that it should be displayed by ls command, >>> but the virtual disk size is okay via qemu-img info. >>> I suspect that during updating l1 table offset, other I/O job reference the big-endian l1 table offset (very large value), >>> so the file is truncated to very large. >> Not quite. Rather, all the data that the snapshot used to occupy is >> still consuming holes in the file; the maximum offset of the file is >> still unchanged, even if the file is no longer using as many referenced >> clusters. Recent changes have gone in to sparsify the file when >> possible (punching holes if your kernel and file system is new enough to >> support that), so that it is not consuming the amount of disk space that >> a mere ls reports. But if what you are asking for is a way to compact >> the file back down, then you'll need to submit a patch. The idea of >> having an online defragmenter for qcow2 files has been kicked around >> before, but it is complex enough that no one has attempted a patch yet. > Sorry, I didn't clarify the problem clearly. > In qcow2_update_snapshot_refcount(), below code, > /* Update L1 only if it isn't deleted anyway (addend = -1) */ > if (ret == 0 && addend >= 0 && l1_modified) { > for (i = 0; i < l1_size; i++) { > cpu_to_be64s(&l1_table[i]); > } > > ret = bdrv_pwrite_sync(bs->file, l1_table_offset, l1_table, l1_size2); > > for (i = 0; i < l1_size; i++) { > be64_to_cpus(&l1_table[i]); > } > } > between cpu_to_be64s(&l1_table[i]); and be64_to_cpus(&l1_table[i]);, > is it possible that there is other I/O reference this interim l1 table whose entries contain the be64 l2 table offset? > The be64 l2 table offset maybe a very large value, hundreds of TB is possible, > then the qcow2 file will be truncated to far larger than normal size. > So we'll see the huge size of the qcow2 file by ls -hl, but the size is still normal displayed by qemu-img info. > > If the possibility mentioned above exists, below raw code may fix it, > if (ret == 0 && addend >= 0 && l1_modified) { > tmp_l1_table = g_malloc0(l1_size * sizeof(uint64_t)) > memcpy(tmp_l1_table, l1_table, l1_size * sizeof(uint64_t)); > for (i = 0; i < l1_size; i++) { > cpu_to_be64s(&tmp_l1_table[i]); > } > ret = bdrv_pwrite_sync(bs->file, l1_table_offset, tmp_l1_table, l1_size2); > > free(tmp_l1_table); > } l1_table is already a local variable (local to qcow2_update_snapshot_refcount()), so I can't really imagine how introducing another local buffer should mitigate the problem, if there is any. Max