[PVE-User] backup to PBS: unable to acquire lock on snapshot directory

[Stephane Caminade <stephane.caminade@ias.u-psud.fr>]

Hello,

I have a question regarding a problem I encounter when running backups 
with PBS.

*Context*:

A cluster of 9 Proxmox nodes running 84 VM, and backing up to a remote PBS

*Versions*:

pve-manager/6.4-13/9f411e79 (running kernel: 5.4.128-1-pve) with 
proxmox-backup-client 1.1.12-1

proxmox-backup-server 2.0.10-1 running version: 2.0.9

*Problem*:

Regularly, some of my VMs (3 regularly, and others randomly - or I have 
not yet determined the common factor -) will fail their backup with the 
following error (example of a regular failing one, but similar message 
for all the failed backups):

/ERROR: Backup of VM 297 failed - VM 297 qmp command 'backup' failed - 
backup connect failed: command error: unable to acquire lock on snapshot 
directory "/mnt/inf-proxmox-bkp/PBS-STORAGE/vm/297/2021-09-29T10:01:43Z" 
- base snapshot is already locked by another operation/


Any pointers as to where I could look for the source of this problem?

Best regards,

Stephane