Fam Zheng <f...@redhat.com> writes: > On Tue, 06/18 08:32, Kevin Wolf wrote: >> 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. > > It's impossible to know a remote protocol points to the same image with > local file path, that's not in QEMU's scope, but we have a good chance > to detect (strcmp with existing bs->filename) and error out Paolo's > A-B-A problem, don't we?
Is comparing bs->filename always a good idea, or only if it's a local image file? If it's a local file, then comparing names to check for aliasing is stupid. Compare device & inode instead.
