On Tue, 06/18 08:37, Markus Armbruster wrote: > Fam Zheng <f...@redhat.com> 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
