Re: [libvirt-users] how "safe" is blockcommit ?

On 09/07/2018 12:06 PM, Lentes, Bernd wrote: > Hi, > > currently i'm following > https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit. I 'm > playing around with it and it seems to be quite nice. > What i want is a daily consistent backup of my image file of the guest. > I have the idea of the following procedure: > > - Shutdown the guest (i can live with a downtime of a few minutes, it will > happen in the night). > And i think it's the only way to have a real clean snapshot Not the only way - you can also use qemu-guest-agent with a trusted guest to quiesce all filesystem I/O after freezing database operations at a consistent point, for a clean snapshot of a live guest. But shutting down is indeed safe, and easier to reason about than worrying whether your qga interaction is properly hooked into all necessary places for a live quiesce. > - create a snapshot with snapshot-create-as: snapshot-create-as guest testsn > --disk-only > - start the guest again. Changes will now go into the overlay, as e.g. inserts > in a database > - rsync the base file to a cifs server. With rsync not the complete, likely big > file is transferred but just the delta We're also trying to add support for incremental backups into a future version of libvirt on top of the qemu 3.0 feature of persistent bitmaps in qcow2 images, which could indeed guarantee that you transfer only the portions of the guest disk that were touched since the last backup. But as that's still something I'm trying to code up, your solution of using rsync to pick out the deltas is as good as anything you can get right now. > - blockcommit the overlay: blockcommit guest /path/to/testsn --active --wait > --verbose --pivot > - delete the snapshot: snapshot-delete guest --snapshotname testsn --metadata > - remove the overlay > > Is that ok ? How "safe" is blockcommit on a running guest ? Yep, that's the right way to do it! It's perfectly safe - the guest doesn't care whether it is reading/writing from the backing file or the overlay, and even if the blockcommit action is aborted, you can restart it for the same effects. (Okay, if you want to get technical, you need to know that committing from 'Base <- mid <- top' down to 'Base' leaves 'mid' in an inconsistent state - but that's not something the guest can see through 'top'; and your specific case is just committing to the immediate backing layer rather than skipping layers like that). > It's possible that during the rsync, when the guest is running, some inserts are > done in a database. As long as the backing file is read-only during the rsync (which it is, since all your guest writes are going to the overlay), then nothing the guest can do will interfere with what rsync can see. Just be sure that you don't start the blockcommit until after rsync is done. > Is it safe to copy the new sectors (i assume that's what blockcommit does) under > a running database ? > Or is it only safe doing blockcommit on a stopped guest ? Live blockcommit is safe, and exists so you don't have to stop the guest. For a bit more insight into what is going on under the hood, the slides from my KVM Forum talk from a few years back may give some nice insights: http://events17.linuxfoundation.org/sites/events/files/slides/2015-qcow2-... > > Thanks for any answer. > > Bernd > > P.S. Is the same procedure possible when the guest disk(s) reside directly in a > plain logical volume, without a file system in-between ? Live blockcommit works onto any host storage protocol (whether filesystem, block device via LVM, or even remote access such as NBD or ceph). The key is that your overlay is a qcow2 file that is tracking the deltas during the time in which you are capturing your backup of the backing file, and then blockcommit safely writes those deltas back into the backing file prior to no longer needing the overlay. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org