From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41149) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XdW8X-0005Yr-1S for qemu-devel@nongnu.org; Sun, 12 Oct 2014 23:17:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XdW8Q-0004xC-QP for qemu-devel@nongnu.org; Sun, 12 Oct 2014 23:17:28 -0400 Received: from [58.251.49.30] (port=38878 helo=mail.sangfor.com) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XdW8Q-0004x1-5z for qemu-devel@nongnu.org; Sun, 12 Oct 2014 23:17:22 -0400 Date: Mon, 13 Oct 2014 11:17:14 +0800 From: "=?utf-8?B?WmhhbmcgSGFveXU=?=" References: <201410091917519618804@sangfor.com>, <201410100954567266628@sangfor.com>, <543A80DA.4090201@redhat.com> Message-ID: <201410131117118042731@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_tableoffset_referenced_by_other_I/O_while_updatin?= =?utf-8?q?g_l1_table_offset_in_qcow2=5Fupdate=5Fsnapshot=5Frefcoun?= =?utf-8?q?t=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. Thanks, Zhang Haoyu >Max