On Thu, Jan 16, 2025 at 16:48:59 -0000, camccuk--- via Users wrote: That document is mostly accurate but it was created before the actual implementation of backups was finished so some things were not actually implemented in the end. The disk quiescing is not part of the backup operation and needs to be done manually via 'virsh domfsfreeze' if required. The original intention was to mirror the snapshot code which does disk quiescing but it's a bit problematic to fold all operations into one so here we didn't do it. Also the backup operation doesn't actually (need to) pause the VM. It can create a point in time backup/copy of the disks without pausing the VM at all. You can even thaw/un-quiesce the disks right away the backup operation starts/while it's running. Yes if you don't override the path in the XML the backup images will be stored in the same path with a suffix of the UNIX timestamp of the time when it was started. By default a full backup creates a stand-alone image. If you'd use incremental backups, then it is actually creating images that depend on each other. Yes it is. Note though that since the VM was likely running at the point when you took the backup the 'restore' operation will look like a cold-boot after a power failure at the exact time when the backup was taken. Snapshots also allow you to capture memory state and also pre-date backups thus they are documented a bit more in depth.

On Fri, Jan 17, 2025 at 00:29:55 -0000, camccuk--- via Users wrote: So normally the quiescing restricts writes to the device and fluses filesystem caches inside the guest OS. In addition the guest agent should allow you to register scripts which are executed before the FS is quiesced allowing e.g. database memory state to be flused to disk so that also the application data is consistent. The application consistency mentioned above is extra important for the use of the backup API or disk-only snapshots as using the saved state is equivalent to pulling out the power plug of a real machine. Yes the guest agent is needed as this operation actually happens inside the guest OS. So the basic 'push' mode of doing a full backup is indeed semantically equivalent of creating a disk-only, no-metadata snapshot, then copying out the data to a standalone image and then merging the overlay back. The backup API though also allows tracking differences since the last backup and creating an incremental backup which would be a thinner image of only the differences. Additionally the backup API also allows PULL mode when an NBD connection to an application doing the backup of the actual blocks is used. So the backup operation is potentially long-running if you're backing up a huge disk. The 'virsh backup-begin' kicks of the operation and returns right away, while the backup progresses on the background. In push mode when qemu is writing the backup image the job is running while data is written, after it finishes an event is fired to clients listening for it notifying that the job is complete and the output images are finished. Note that the state of the backup will still correspond to the point in time when the operation was *started*, even when the guest OS overwrites any blocks subsequently. For a pull mode backup the client doing the backup knows when it's ready so the job is not auto-finished (which would fire the event) but rather needs to be terminated manually. Yes exactly, currently only snapshots allow memory state capture synchronized with disk state capture. A (managed)-save saves only the memory state to an image, disk images are kept as they are. No preservation points for the disks are created. Resuming from the (managed)-save will continue using/modifying the disk image without the possibility of getting back. It is indeed meant to e.g. preserve the state of VM while the host OS reboots. 'managed' is in brackeds as there is also a non-managed save.