[libvirt] server side copy from one volume to another

[Daniel P. Berrange]

On Tue, Apr 18, 2017 at 01:31:11PM +0200, Michal Privoznik wrote:
> On 04/18/2017 01:03 PM, Vasiliy Tolstov wrote:
> > 2017-04-18 8:39 GMT+03:00 Michal Privoznik <mprivozn at redhat.com>:
> > > s that right? Here I'm successfully cloning a volume across two storage pools:
> > 
> > 
> > Yes, it works but....
> > Source image:
> > image: /var/lib/libvirt/images/sda/143177
> > file format: qcow2
> > virtual size: 5.0G (5368709120 bytes)
> > disk size: 902M
> > cluster_size: 65536
> > Format specific information:
> >     compat: 0.10
> >     refcount bits: 16
> > 
> > Dest image:
> > image: /var/lib/libvirt/images/sda/test
> > file format: qcow2
> > virtual size: 5.0G (5368709120 bytes)
> > disk size: 902M
> > cluster_size: 65536
> > Format specific information:
> >     compat: 0.10
> >     refcount bits: 16
> > 
> > 
> > But
> > -rw------- 1 root 908M Apr 18 13:59 143177
> > -rw------- 1 root 902M Apr 18 13:59 test
> > 
> > Why size is changed on destination image? In case of cp
> > --spwarse=always size does not changed.
> > 
> 
> That's because libvirt does not use `cp` to copy the disk images. We use
> qemu-img for copying. And we do not guarantee that the disk structure does
> not change. We guarantee that the data stored on the disk doesn't change.
> Otherwise we wouldn't be able to convert disk images.
> 
> NB, with sparse files - it's not as straightforward either. Some filesystems
> may decide that the hole is not big enough so they do allocate it actually.
> XFS is a very good example of such behaviour. So the data/hole structure of
> a file may not line up with the structure of a copy. Although, the data as
> read from the files are the same.

XFS will also do speculative allocation of filesystem blocks which have
not yet had data written to them, on the basis that a future write might
need them soon. So you can't really compare allocation disk size at all.

Regards,
Daniel
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