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. [...]
