From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34014) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XdarP-0000xB-9E for qemu-devel@nongnu.org; Mon, 13 Oct 2014 04:20:13 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XdarJ-00071g-22 for qemu-devel@nongnu.org; Mon, 13 Oct 2014 04:20:07 -0400 Received: from [58.251.49.30] (port=33973 helo=mail.sangfor.com) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XdarI-00070V-2r for qemu-devel@nongnu.org; Mon, 13 Oct 2014 04:20:00 -0400 Date: Mon, 13 Oct 2014 16:19:50 +0800 From: "=?utf-8?B?WmhhbmcgSGFveXU=?=" References: <201410091917519618804@sangfor.com>, <201410100954567266628@sangfor.com>, <543A80DA.4090201@redhat.com>, <201410131117118042731@sangfor.com>, <543B73F1.3090907@redhat.com>, <201410131513365349242@sangfor.com>, <543B872C.4090104@redhat.com> Message-ID: <201410131619475305762@sangfor.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] =?utf-8?q?=5Bquestion=5D_is_it_possible_that_big-end?= =?utf-8?q?ian_l1_tableoffsetreferencedby_other_I/O_while_updating_?= =?utf-8?q?l1_table_offset_in_qcow2=5Fupdate=5Fsnapshot=5Frefcount?= =?utf-8?q?=3F?= List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: =?utf-8?B?TWF4IFJlaXR6?= , =?utf-8?B?RXJpYyBCbGFrZQ==?= , =?utf-8?B?cWVtdS1kZXZlbA==?= Cc: =?utf-8?B?S2V2aW4gV29sZg==?= , =?utf-8?B?U3RlZmFuIEhham5vY3pp?= >>>>>>>> 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. > Yes, bs->file is passed to bdrv_pwrite_sync here, but aio_poll(aio_context) will poll all BDS's aio, not only that of bs->file, doesn't it? Is it possible that there are pending aio which belong to this qcow2 BDS still exist? Thanks, Zhang Haoyu >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