On Fri, May 01, 2020 at 16:05:47 +0100, Stefan Hajnoczi wrote: The approaches have diffrerent kind of data integrity they provide and downtime they require. Assuming from the above that you don't want to shutdown the OS in the VM you've got following options: 1) pause VM and copy images as described above I definitely don't recommend this approach at all. The drawback is that the QCOW2 file on the disk may have inconsistent metadata. Even if you ensure that the gues OS state is consistend and written to disk it's not guaranteed that qemu's buffers were flushed. Also the VM needs to be suspended during the whole copy, unless you have the image on a filesystem which has --reflink support as pointed out by Stefan. 2) 'virsh blockcopy' The original idea of blockcopy is to move storage of an active VM to a different location. It can be used though to "copy" the active disk and ensure that the metadata is correct when combined with 'virsh blockjob --abort' to finish it. This still requires the guest OS in the VM to ensure that the filesystems on the backed-up disk are consistent. Also the API has one very severe limitation if your VM has multiple disks: There is no way to ensure that you cancel all the copies at the same time, so the 'backup' done this way is not taken at a single point in time. It's also worth noting that the point in time the backup is taken is when the job is --abort-ed. 3) virsh backup This is the newest set of APIs specifically designed to do disk backups of the VM, offers consistency of the image metadata, and taking of the backups of multiple disks at the same point in time. Also the point in time is when the API is started, regardless of how long the actual data handling takes. Your gues OS still needs to ensure filesystem consistency though. Additionally as mentioned by Stefan below you can also do incremental backups as well. One thing to note though is that the backup integration is not entirely finished in libvirt and thus in a 'tech-preview' state. Some interactions corrupt the state for incremental backups. If you are interested, I can give you specific info how to enable support for backups as well as the specifics of the current state of implementation. 4) snapshots Libvirt's snapshot implementation supports taking full VM snapshots including memory and disk image state. This sidesteps the problem of inconsistent filesystem state as the memory state contains also all the buffers. When an external snapshot is created, we add a new set of overlay files on top of the original disk images. This means that they become effectively read-only. You can then copy them aside if you want so. The memory image taken along can be then used to fully restore the state of the VM. There are a few caveats here as well. If the image chain created this way becomes too long it may negatively impact performance. Also reverting the memory image is a partially manual operation for now. I can give specifics if you want.

On Fri, May 01, 2020 at 04:05:47PM +0100, Stefan Hajnoczi wrote: Hi Anders, tl;dr: While 'blockcopy' is one way to do a full backup; there's a better way: 'blockcommit'; see below for a URL to an example. To add to what Stefan says below, here's the difference: a regular 'cp' is just that — an offline copy when your guest is shutdown or suspended. However, libvirt's 'blockcopy' lets you create a "point-in-time snapshot [or copy]" of your VM's disk _while_ the VM is running, and optionally, lets your VM to "live-pivot" its storage to the just-created copy. The use-case is "live storage migration". Say, your original storage of your VM is on NFS-A, but you want to do some maintenance on NFS-A. Here, 'blockcopy' lets you live-copy the VM's storage from NFS-A to NFS-B, _and_ make the VM use the copy — all this without causing any downtime to the users of the VM. Now you can freely do the maintenance on NFS-A. A 'blockcopy' operation has two phases: (1) It copies a VM's disk image, e.g. from 'orig.raw' to 'copy.qcow2', while the VM is running. And it will _keep_ copying as long as your VM keeps writing new data. This is called the "mirroring" phase. (2) Once the copy.qcow2 has the same content as orig.raw, then you can do two things, either (a) end the copying/mirroring, which results in a point-in-time 'snapshot' of the orig.raw; or (b) you can "live-pivot" the VM's disk image to just-created copy.qcow2. Management tools like OpenStack (and possibly others) use libvirt's 'blockcopy' API under the hood to allow live storage migration. There are other useful details here, but I'll skip them for brevity. Read the "blockcopy" section in `man virsh`. I admit it can be a little hard to parse when you normally don't dwell on these matters, but taking time to experiement gives a robust understanding. If your main goal is to take a full backup of your disk, *without* any downtime to your VM, libvirt does provide some neat ways. One of the most efficient methods to is the so-called "active block-commit". It uses a combination of libvirt commands: `virsh snapshot-create-as`, `virsh blockcommit`, and `rsync`: I've written up a full example here: https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit You might also want to refer to this page. https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit - - - For libvirt-based incremental backup examples, I'll defer that to Eric Blake (in Cc). [...] -- /kashyap