Fam Zheng <f...@redhat.com> writes: > On Tue, 06/18 16:18, Markus Armbruster wrote: >> 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? > > It's never sufficient by comparing filename to tell if they are the > same, things can be tricky here, but in many cases it can be helpful, > both local and remote.
Let me rephrase my question. We all understand that different bs->filename can alias the same resource (which is not necessarily a file). This makes a "same resource" test based on bs->filename incomplete. Does identical bs->filename *always* imply same resource? If yes, the test is correct but incomplete. That can be useful. If no, the test is incorrect and incomplete, thus useless. >> If it's a local file, then comparing names to check for aliasing is >> stupid. Compare device & inode instead. > > Device and inode is not something to block layer's knowledge, I think. They are one stat(2) or fstat(2) away.
