Re: Backup of vm disk images

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