Domain backup file explodes on s3fs

[Tim Haley]

Hi all,

Have been playing with `virsh backup-begin` of late and think it's an 
excellent feature. I've noticed one behavior I'm not sure I understand. 
Am doing pretty straight-forward backup of boot disk:

# cat bx
<domainbackup>
  <disks>
    <disk name='vda' type='file'>
    <target file='/backups/vda.2aa450cc-6d2e-11ea-8de0-52542e0d008a'/>
      <driver type='qcow2'/>
    </disk>
  </disks>
</domainbackup>

# cat cx
<domaincheckpoint>
  <name>2aa450cc-6d2e-11ea-8de0-52542e0d008a</name>
  <disks>
    <disk name='vda' checkpoint='bitmap'/>
  </disks>
</domaincheckpoint>

# virsh backup-begin 721 bx cx

If my /backups directory is just XFS, I get a backup file that looks 
like it is just the size of data blocks in use

-rw------- 1 root  root  2769551360 Mar 19 16:56 
vda.2aa450cc-6d2e-11ea-8de0-52542e0d008a

but if I write to an s3fs (object storage backend) the file blows up to 
the whole size of the disk

-rw------- 1 root  root  8591507456 Mar 18 19:03 
vda.2aa450cc-6d2e-11ea-8de0-52542e0d008a

Is this expected?

If it's relevant, this is on ubuntu and
# virsh version
Compiled against library: libvirt 6.1.0
Using library: libvirt 6.1.0
Using API: QEMU 6.1.0
Running hypervisor: QEMU 4.2.0

thanks for any ideas,
-tim