On Tue, 06/18 09:51, Kevin Wolf wrote: > Am 18.06.2013 um 09:00 hat Fam Zheng geschrieben: > > 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. In this case do you mean mgmt tool should give a name of drive instead of file path? I like this idea, and further more, why don't we make QEMU smarter to bdrv_find_by_filename() the existing BDS? > > > > > > > > > > > > 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? > > Yes, catching 50% of the misuses is better than catching none. > > My point was that we shouldn't "try to reuse the bs" when we detect that > the file is already open, because that makes it a feature that users are > supposed to use and that doesn't work consistently across backends and > will therefore cause endless pain.
OK. > > If we detect it (in order to protect the user from his own mistakes), we > must treat it as a misuse and return an error. > IIUC, block job is not supposed to affect the guest or the source image, so from user's PoV, switching to another image, then switching back seems reasonable, even when a block job runs in the background. As we know it's already open, could we reattach to it instead, as you suggested above? -- Fam
