Kevin Wolf <kwolf@xxxxxxxxxx> writes: > Am 06.04.2023 um 22:23 hat Reinoud Zandijk geschrieben: >> On Tue, Apr 04, 2023 at 06:17:45PM +0200, Kevin Wolf wrote: >> > Am 04.04.2023 um 17:07 hat Michael Tokarev geschrieben: >> > > 04.04.2023 16:57, Kevin Wolf пишет: >> > Maybe -snapshot should error out if -blockdev is in use. You'd generally >> > expect that either -blockdev is used primarily and snapshots are done >> > externally (if the command line is generated by some management tool), >> > or that -drive is used consistently (by a human who likes the >> > convenience). In both cases, we wouldn't hit the error path. >> > >> > There may be some exceptional cases where you have both -drive and >> > -blockdev (maybe because a human users needs more control for one >> > specific disk). This is the case where you can get a nasty surprise and >> > that would error out. If you legitimately want the -drive images >> > snapshotted, but not the -blockdev ones, you can still use individual >> > '-drive snapshot=on' options instead of the global '-snapshot' (and the >> > error message should mention this). >> >> I didn't know that! I normally use the -snapshot as global option. Is there a >> reason why -blockdev isn't honouring -snapshot? > > The philosophy behind -blockdev is that you're explicit about every > image file (and other block node) you want to use and that QEMU doesn't > magically insert or change things behind your back. > > For simple use cases that might not seem necessary, but many of the > newer functions added to the block layer, like the block jobs, are > operations that can work on any node in the block graph (i.e. any of the > open images, including backing files etc.). If QEMU changed something > behind your back, you can easily access the wrong image. Especially for > management software like libvirt this kind of magic that -drive involves > was really hard to work with because it always had to second guess what > the world _really_ looked like on the QEMU side. > > For example, imagine you open foo.img with -snapshot. Now you want to > create a backup of your current state, so tell QEMU to backup the block > node for foo.img because that's what your VM is currently running on, > right? Except that nobody told you that the active image is actually a > temporary qcow2 image file that -snapshot created internally. You're > backing up the wrong image without the changes of your running VM. > > So it's better to always be explicit, and then it's unambiguous which > image file you really mean in operations. With that in mind please review: Subject: [PATCH v3] qemu-options: finesse the recommendations around -blockdev Date: Thu, 6 Apr 2023 10:53:17 +0100 Message-Id: <20230406095317.3321318-1-alex.bennee@xxxxxxxxxx> -- Alex Bennée Virtualisation Tech Lead @ Linaro
