I have my Promox to backup the containers and virtual machines on a fixed schedule. I am using an NFS storage as my backup target. I recently encountered this failed container backups not too long ago. The error I was getting is below.
Feb 18 21:25:20 pve2 pvedaemon[21419]: ERROR: Backup of VM 104 failed - command 'set -o pipefail && lxc-usernsexec -m u:0:100000:65536 -m g:0:100000:65536 -- tar cpf - --totals --one-file-system -p --sparse --numeric-owner --acls --xattrs '--xattrs-include=user.*' '--xattrs-include=security.capability' '--warning=no-file-ignored' '--warning=no-xattr-write' --one-file-system '--warning=no-file-ignored' '--directory=/mnt/pve/darktower_backup/dump/vzdump-lxc-104-2021_02_18-21_25_05.tmp' ./etc/vzdump/pct.conf ./etc/vzdump/pct.fw '--directory=/mnt/vzsnap0' --no-anchored '--exclude=lost+found' --anchored '--exclude=./tmp/?*' '--exclude=./var/tmp/?*' '--exclude=./var/run/?*.pid' ./ | zstd --rsyncable '--threads=1' >/mnt/pve/darktower_backup/dump/vzdump-lxc-104-2021_02_18-21_25_05.tar.dat' failed: exit code 2
To fix the issue, editing the /etc/vzdump.conf file did the trick for me. I found the solution in the Proxmox forum after looking up the error. The questionable line is the #tmpdir: DIR. This line need to be changed.
# Original #tmpdir: DIR # Updated tmpdir: /tmp
My understanding with this is Proxmox requires a tmpdir for compression, but somehow could not access the temporary folder. I could be wrong, but regardless the backup works again for the LXC containers.
Cheers!