From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59619) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Xdaaw-0006AM-Jv for qemu-devel@nongnu.org; Mon, 13 Oct 2014 04:03:12 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Xdaaq-0000Zp-AK for qemu-devel@nongnu.org; Mon, 13 Oct 2014 04:03:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45046) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Xdaaq-0000Zd-2f for qemu-devel@nongnu.org; Mon, 13 Oct 2014 04:03:00 -0400 Message-ID: <543B872C.4090104@redhat.com> Date: Mon, 13 Oct 2014 10:02:52 +0200 From: Max Reitz MIME-Version: 1.0 References: <201410091917519618804@sangfor.com>, <201410100954567266628@sangfor.com>, <543A80DA.4090201@redhat.com>, <201410131117118042731@sangfor.com>, <543B73F1.3090907@redhat.com> <201410131513365349242@sangfor.com> In-Reply-To: <201410131513365349242@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 tableoffsetreferenced 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 Cc: Kevin Wolf , Stefan Hajnoczi Am 13.10.2014 um 09:13 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. >>>> >>> l1_table is not necessarily a local variable to qcow2_update_snapshot_refcount, >>> which depends on condition of "if (l1_table_offset != s->l1_table_offset)", >>> if the condition not true, l1_table = s->l1_table. >> Oh, yes, you're right. Okay, so in theory nothing should happen anyway, >> because qcow2 does not have to be reentrant (so s->l1_table will not be >> accessed while it's big endian and therefore possibly not in CPU order). > Could you detail how qcow2 does not have to be reentrant? > In below stack, > qcow2_update_snapshot_refcount > |- cpu_to_be64s(&l1_table[i]) > |- bdrv_pwrite_sync This is executed on bs->file, not the qcow2 BDS. Max > |-- bdrv_pwrite > |--- bdrv_pwritev > |---- bdrv_prwv_co > |----- aio_poll(aio_context) <== this aio_context is qemu_aio_context > |------ aio_dispatch > |------- bdrv_co_io_em_complete > |-------- qemu_coroutine_enter(co->coroutine, NULL); <== coroutine entry is bdrv_co_do_rw > bdrv_co_do_rw will access l1_table to perform I/O operation. > > Thanks, > Zhang Haoyu >> But I find it rather ugly to convert the cached L1 table to big endian, >> so I'd be fine with the patch you proposed. >> >> Max