From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46102) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Uoq08-0007Ey-1V for qemu-devel@nongnu.org; Tue, 18 Jun 2013 03:06:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Uoq04-0005SR-Ra for qemu-devel@nongnu.org; Tue, 18 Jun 2013 03:06:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:15737) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Uoq04-0005P5-Jm for qemu-devel@nongnu.org; Tue, 18 Jun 2013 03:06:44 -0400 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r5I76gqg026462 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Tue, 18 Jun 2013 03:06:43 -0400 Date: Tue, 18 Jun 2013 15:06:41 +0800 From: Fam Zheng Message-ID: <20130618070641.GB1220@localhost.nay.redhat.com> References: <51BED513.3030800@redhat.com> <20130617093241.GA22609@localhost.nay.redhat.com> <51BEDCB9.5090905@redhat.com> <20130617135253.GB3994@dhcp-200-207.str.redhat.com> <51BF16B8.6040801@redhat.com> <20130617142605.GD3994@dhcp-200-207.str.redhat.com> <51BF213F.60601@redhat.com> <20130617151238.GF3994@dhcp-200-207.str.redhat.com> <20130618035854.GA17533@localhost.nay.redhat.com> <871u7zx6u3.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <871u7zx6u3.fsf@blackfin.pond.sub.org> Subject: Re: [Qemu-devel] [PATCH] block: add 'backing' option to drive_add List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: Kevin Wolf , Paolo Bonzini , qemu-devel@nongnu.org, stefanha@redhat.com On Tue, 06/18 08:37, Markus Armbruster wrote: > Fam Zheng writes: > > > On Mon, 06/17 17:12, Kevin Wolf wrote: > >> Am 17.06.2013 um 16:46 hat Paolo Bonzini geschrieben: > >> > Il 17/06/2013 16:26, Kevin Wolf ha scritto: > >> > > Am 17.06.2013 um 16:01 hat Paolo Bonzini geschrieben: > >> > >> Il 17/06/2013 15:52, Kevin Wolf ha scritto: > >> > >>> It's not a new thought that we need to change the block layer so that a > >> > >>> BlockDriverState can't be "empty", but that one BlockDriverState always > >> > >>> refers to one image. If you change media, you attach a different > >> > >>> BlockDriverState to the device. Once you have this, you can start > >> > >>> refcounting BlockDriverStates, so that the backing file remains usable > >> > >>> while the guest device already uses a different image. > >> > >>> > >> > >>> Not that it's it easy to get there... > >> > >> > >> > >> I'm not sure that is safe to do. > >> > >> > >> > >> Consider the case where the guest switches from A to B during backup, > >> > >> and then from B to A. You get two BDS for the same file, which pretty > >> > >> much means havoc. > >> > > > >> > > Well, yes, it means that the management tool needs to know what it's > >> > > doing. It shouldn't create a second BDS for A, but reattach the still > >> > > existing one. > >> > > >> > How? That would require the management tool to know the full chain of > >> > BDSes that were opened in the past. > >> > >> They better know on which files they are operating. It's not like the > >> management could be unaware of running backup jobs or things like that. > >> > > > > Is there any case that QEMU needs to have two BDS pointing to the same > > file? > > Maybe, I don't know. > > > If not, can we try to detect such case on opening > > Gee, what a nice swamp you found there! > > For local files, you can compare (dev-major, dev-minor, inode). > > Beyond that, you tend not to get comparisons, but best guesses. > > > and try to > > reuse the bs? > > I doubt reusing the BDS is correct in the general case. > Maybe I meant basically the same as Kevin, but just that QEMU finds out A has existing BDS, and reattach it. > >> > > Well, yes, it means that the management tool needs to know what it's > >> > > doing. It shouldn't create a second BDS for A, but reattach the still > >> > > existing one. I assume it should be less wrong than having two BDS for the same file. -- Fam