Export checkpoint/bitmap from image on qcow2

[João Jandre Paraquetti]

Hi, all

I recently started looking into how to create incremental backups using 
Libvirt+Qemu. I have already found that, since Libvirt 7.6.0, we can use 
the virDomainBackupBegin API to create incremental backups of live VMs 
through Libvirt. Using this API we can back up the VM's disks and create 
a checkpoint at the same time. After the first full backup, we can 
create incremental backups referencing the checkpoint that was generated 
on the previous backup process.

As far as I understood Libvirt's documentation, when using this API to 
create a checkpoint, what happens is that Libvirt will create a new 
bitmap on the VM's disk to track the changes that happened from that 
checkpoint on, then, they'll copy what changed between the last bitmap 
and the new one. By doing this, Libvirt is able to create incremental 
backups of disks that are used by running VMs.

My problem is the following though: after I stop the VM, I'm no longer 
able to use Libvirts API as it requires the VM (domain) to be running. 
However, there might still be changes on the disk since the last backup 
I performed. Therefore, to create a backup of the changes made since the 
last backup until the VM was stopped, I have to work directly on the VM 
image using Qemu. I have checked, and the QCOW2 file has the checkpoint 
created with the Libvirt backup process (I checked via the "qemu-img 
info"). Therefore, we have the marker of the last checkpoint generated 
by the last backup process. However, if the VM is stopped, there is no 
clear way to export this last differencial copy of the volume.

I have searched through the documentation about this; however, I haven't 
found any example or explanation on how to manually export a 
differencial copy of the QCOW2 file using the last checkpoint (bitmap) 
created.

I would much appreaciate, if people could point me to the direction on 
how to export differencial copies of a QCOW2 file, using the checkpoints 
(bitmaps) that it has.

Thank you guys in advance!