virsh backup-begin problem

[Peter Krempa]

On Thu, Jan 21, 2021 at 13:31:03 +0300, Andrey Fokin wrote:
> Thanks Peter!
> But is possible change XML configuration towards will be possible to do
> full (push) backup?

Yes, I've included links to the documentation to do so in my previous
reply, so I'd suggest you follow it. See ...

> On Wed, Jan 20, 2021 at 9:08 PM Peter Krempa <pkrempa at redhat.com> wrote:
> 
> > On Wed, Jan 20, 2021 at 20:49:49 +0300, Andrey Fokin wrote:
> > > Colleagues, could you please advise what a problem could be in XML file
> > in
> > > case of:
> > > virsh backup-begin lubuntu2 ./backup.xml
> > >
> > > where backup.xml -
> > > <domainbackup>
> > >     <disk name='/var/lib/libvirt/images/lubuntu2.qcow2' type='file'>
> > >     <target file='$PWD/scratch1.img'/>
> > >     <driver type='raw'/>
> > >     </disk>
> > >     <disk name='/var/lib/libvirt/images/lubuntu2-1.qcow2' type='file'>
> > >       <target file='$PWD/scratch2.img'/>
> > >       <driver type='raw'/>
> > >     </disk>
> > > </domainbackup>
> >
> > snip ...
> >
> > You forgot to add what you are observing, but I presume:
> >
> > 'operation unsupported: incremental backup is not supported yet'
> >
> > Unfortunately it's what it says on the tin. The implementation of the
> > incremental backup code is not finished yet, thus the feature is
> > disabled.
> >
> > If you want to test-drive incremental backup APIs, you can enable the
> > experimental feature by using the XML documented in:
> >
> > https://libvirt.org/drvqemu.html#xmlnsfeatures

... here. I specifically avoided giving exact steps so that you read the
documentation including the description of caveats of enabling this
_experimental_ feature.

So please make sure to read it. This is NOT meant for production use.

Anyways:

You need to add the 'qemu' namespace to your domain XML:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>

And force-on the incremental backup capability:

  <qemu:capabilities>
    <qemu:add capability='incremental-backup'/>
  </qemu:capabilities>


This enables backups, but as I've wrote previously ...

> >
> > to enable a feature named 'incremental-backup'. Please make sure to read
> > the caveats of doing so thoroughly.

.. observe the following:

> > I also advise not to combine incremental backups with snapshots
> > (virDomainSnapshot* APIs) and blockjobs (virDomainBlock* APIs) as that
> > will not work properly without a patch enabling also experimental qemu
> > features.

If you use them, backups will break. You get to keep the pieces, as
you've been warned.

Also this:

> > I also suggest you use the latest qemu for this.