LXC Backup fails due to read error

[DaanSelen]

Hello all, I have a proxmox setup, which is pretty basic. Other than maybe some VirGL packages.
However I get this error when I am trying to backup my LXC container:

INFO: starting new backup job: vzdump --notes-template '{{guestname}}' --mailnotification failure --mode snapshot --node prox1 --mailto dselen@systemec.nl --all 1 --notification-mode legacy-sendmail --fleecing 0 --storage QNAP-NFS-THIN --compress zstd
INFO: Starting Backup of VM 100 (lxc)
INFO: Backup started at 2025-10-30 08:49:49
INFO: status = running
INFO: CT Name: nginxproxymanager
INFO: including mount point rootfs ('/') in backup
INFO: mode failure - some volumes do not support snapshots
INFO: trying 'suspend' mode instead
INFO: backup mode: suspend
INFO: ionice priority: 7
INFO: CT Name: nginxproxymanager
INFO: including mount point rootfs ('/') in backup
INFO: temporary directory is on NFS, disabling xattr and acl support, consider configuring a local tmpdir via /etc/vzdump.conf
INFO: starting first sync /proc/1642727/root/ to /mnt/pve/QNAP-NFS-THIN/dump/vzdump-lxc-100-2025_10_30-08_49_49.tmp
INFO: first sync finished - transferred 642.38M bytes in 44s
INFO: suspending guest
INFO: starting final sync /proc/1642727/root/ to /mnt/pve/QNAP-NFS-THIN/dump/vzdump-lxc-100-2025_10_30-08_49_49.tmp
INFO: final sync finished - transferred 0 bytes in 1s
INFO: resuming guest
INFO: guest is online again after 1 seconds
INFO: creating vzdump archive '/mnt/pve/QNAP-NFS-THIN/dump/vzdump-lxc-100-2025_10_30-08_49_49.tar.zst'
INFO: tar: ./var/cache/apt/archives/partial: Cannot open: Permission denied
INFO: tar: ./var/lib/apt/lists/partial: Cannot open: Permission denied
INFO: tar: ./var/lib/postfix/master.lock: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/active: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/bounce: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/corrupt: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/defer: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/deferred: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/flush: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/hold: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/incoming: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/maildrop: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/private: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/public: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/saved: Cannot open: Permission denied
INFO: tar: ./var/spool/postfix/trace: Cannot open: Permission denied
INFO: Total bytes written: 670648320 (640MiB, 47MiB/s)
INFO: tar: Exiting with failure status due to previous errors
ERROR: Backup of VM 100 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/QNAP-NFS-THIN/dump/vzdump-lxc-100-2025_10_30-08_49_49.tmp' ./etc/vzdump/pct.conf ./etc/vzdump/pct.fw '--directory=/mnt/pve/QNAP-NFS-THIN/dump/vzdump-lxc-100-2025_10_30-08_49_49.tmp' --no-anchored '--exclude=lost+found' --anchored '--exclude=./tmp/?*' '--exclude=./var/tmp/?*' '--exclude=./var/run/?*.pid' . | zstd '--threads=1' >/mnt/pve/QNAP-NFS-THIN/dump/vzdump-lxc-100-2025_10_30-08_49_49.tar.dat' failed: exit code 2
INFO: Failed at 2025-10-30 08:50:53

How does this happen?

 

[Magnus-mercer]

On NFS the exported filesystem often uses root_squash (or simply stores numeric UIDs) so the mapped process has no permission to read files that are owned by numeric 0.