PBS self-backup fail and success

[logui]

I am running PBS as a VM in Proxmox, I have a cluster with 3 nodes, and PBS in running on one of them, I have an external USB drive with USB passthrough to the VM, everything works fine, backing up all the different VMs across all nodes in the cluster.

Today I tried to backup the PBS VM, I know, it sounds non-sense, but I wanted to try, in theory If the backup process takes a Snapshot of the VM without doing anything to it, it should work.

Initially it failed when issuing the quest-agent 'fs-freeze' command, that makes sense, because while backing up the PBS VM, itself (PBS VM) received an instruction to freeze itself, and that broke the backup process, no issues here.

Then I decided to remove the qemu-guest-agent from the PBS VM and try again, in this scenario the backup of the PBS VM on PBS worked fine, because a Snapshot was taken without impacting the running PBS VM.

So, my question is, please could you explain what is happening here? Are my assumptions (as described above) correct? Is everything working as per design? Should I do it differently? Thank you

 

[UdoB]

If you really believe double posting is fine (which it usually is not) you should at least cross reference them... --> https://forum.proxmox.com/threads/pbs-self-backup-fail-and-success.158364/

 

[fabian]

for VMs, there is no actual snapshot that is being taken - Qemu is instructed to handle writes differently, intercepting them before hitting the guest volumes. how exactly this works depends on the guest and backup settings, but backing up a VM that is itself the backup target doesn't work, because there is a short time at the start of the backup where the VM's I/O is basically paused or even execution suspended entirely.

 

[leesteken]

Slightly off-topic: a PBS that runs in a (unprivileged) container can backup itself just fine (as long as the datastore is separate and not visible to Proxmox).

 

[fabian]

yes, container backup and VM backup work very differently. for containers it does also depend on the backup mode though - a stop mode backup will still be rather hard