From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40009) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UopTQ-0000jX-Us for qemu-devel@nongnu.org; Tue, 18 Jun 2013 02:33:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UopTP-0002TD-Uk for qemu-devel@nongnu.org; Tue, 18 Jun 2013 02:33:00 -0400 Received: from mx1.redhat.com ([209.132.183.28]:5366) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UopTP-0002T3-Lr for qemu-devel@nongnu.org; Tue, 18 Jun 2013 02:32:59 -0400 Received: from int-mx12.intmail.prod.int.phx2.redhat.com (int-mx12.intmail.prod.int.phx2.redhat.com [10.5.11.25]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r5I6Wwnq017994 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Tue, 18 Jun 2013 02:32:59 -0400 Date: Tue, 18 Jun 2013 08:32:56 +0200 From: Kevin Wolf Message-ID: <20130618063256.GB3640@dhcp-200-207.str.redhat.com> References: <1371457366-10993-1-git-send-email-famz@redhat.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130618035854.GA17533@localhost.nay.redhat.com> 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: Paolo Bonzini , qemu-devel@nongnu.org, stefanha@redhat.com, Eric Blake , armbru@redhat.com Am 18.06.2013 um 05:58 hat Fam Zheng geschrieben: > 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? No, I think there's no case where this would make sense. > If not, can we try to detect such case on opening and try to > reuse the bs? We can't do it reliably, think about symlinks or even hard links, or things like /dev/fdset/..., let alone remote protocols that refer to the same image file etc. We can check the obvious cases and error out for them, but that's about what we can do. I don't think we should try to fix things automagically when we can't do it right. Kevin