libvirt users - Jan 2022 - What does the positional parameters of "virsh backup-begin" actually do?

Normally when I backup a kvm machine I shutdown the machine then run:

virsh list -all
virsh shutdown Ubuntu18
virsh dumpxml Ubuntu18 > /MyBackup/Ubuntu18.xml
cp /var/lib/libvirt/images/Ubuntu18.qcow2 /MyBackup/Ubuntu18.qcow2

However, I found a new sub-command. The help file says:

% virsh backup-begin --help
  NAME
    backup-begin - Start a disk backup of a live domain

  SYNOPSIS
    backup-begin <domain> [--backupxml <string>] [--checkpointxml
<string>] [--reuse-external]

  DESCRIPTION
    Use XML to start a full or incremental disk backup of a live
domain, optionally creating a checkpoint

  OPTIONS
    [--domain] <string>  domain name, id or uuid
    --backupxml <string>  domain backup XML
    --checkpointxml <string>  domain checkpoint XML
    --reuse-external  reuse files provided by caller

The problem with this help is, it is not clear enough.

I understand that I should use virsh backup-begin vm1 to backup a live kvm
machine. However, this command only create .qcow2 files. What about the .xml
file.

What does --backupxml , --checkpointxml & --reuse-external actually do?

When should I use them?
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On Fri, Jan 14, 2022 at 23:39:10 +0600, Ahmad Ismail
wrote:
> Normally when I backup a kvm machine I shutdown the machine then run:
[...]
> The problem with this help is, it is not clear enough.
> 
> I understand that I should use virsh backup-begin vm1 to backup a live kvm
> machine. However, this command only create .qcow2 files. What about the
.xml
> file.
> 
> What does --backupxml , --checkpointxml & --reuse-external actually do?
> 
> When should I use them?
'man virsh' states:

   backup-begin
       Syntax:

          backup-begin domain [backupxml] [checkpointxml] [--reuse-external]

       Begin a new backup job. If backupxml is omitted, this defaults  to
       a  full  backup  using a push model to filenames generated by lib?
       virt; supplying XML allows fine-tuning such as requesting  an  in?
       cremental  backup  relative  to an earlier checkpoint, controlling
       which disks participate or which filenames are  involved,  or  re?
       questing  the use of a pull model backup.  The backup-dumpxml com?
       mand shows any resulting values assigned by libvirt. For more  in?
       formation           on          backup          XML,          see:
       https://libvirt.org/formatbackup.html

       If --reuse-external is used it instructs libvirt to  reuse  tempo?
       rary and output files provided by the user in backupxml.

       If checkpointxml is specified, a second file with a top-level ele?
       ment of domaincheckpoint is used to create a  simultaneous  check?
       point,  for  doing a later incremental backup relative to the time
       the backup was created. See checkpoint-create for more details  on
       checkpoints.

       This  command returns as soon as possible, and the backup job runs
       in the background; the progress of a  push  model  backup  can  be
       checked with domjobinfo or by waiting for an event with event (the
       progress of a pull model backup is under the control  of  whatever
       third  party  connects  to  the NBD export). The job is ended with
       domjobabort.


Does this clarify it sufficiently?