← All break/fix scenarios in Proxmox VE
PBS backup fails with "permission denied" writing chunks
Reported symptoms
- ●PBS backup job fails with "Permission denied" on chunk creation
- ●Task log shows EACCES writing to /datastore/.../chunks
- ●Other PBS jobs (sync, prune, GC) succeed
- ●A new datastore or recent migration is involved
Evidence
- · ls -la /datastore/<store>/chunks shows root-owned files but PBS runs as backup
- · ps aux | grep proxmox-backup-proxy shows the user
- · id backup (PBS service user)
- · mount shows the datastore filesystem read-only
Diagnosis and resolutionclick to reveal
Root cause
Either the PBS service user lacks write permission to the chunk directory (often after a manual restore or migration), or the underlying filesystem is mounted read-only. The latter usually indicates filesystem corruption that the kernel has remounted ro to protect data.
Remediation
1. Check the filesystem state: `mount | grep datastore` (look for "ro,") 2. If read-only: - Unmount: `umount /datastore/<store>` - Run fsck: `fsck -n /dev/<device>` (dry run first) - If clean: `mount -o rw /datastore/<store>` - Investigate why it went read-only (`dmesg`) 3. If permission issue: `chown -R backup:backup /datastore/<store>/chunks` `chmod 700 /datastore/<store>/chunks` 4. Restart PBS: `systemctl restart proxmox-backup-proxy` 5. Retry the backup.
Verification
- `mount | grep datastore` shows `rw,` - `ls -la /datastore/<store>/chunks` shows backup:backup ownership - Manual PBS backup completes successfully - Scheduled job runs and produces a fresh snapshot
Prevention
- Use PBS's built-in datastore creation, not manual `mkdir` - Monitor the underlying disk health with SMART - Configure email alerts for read-only mounts via udev rules - Run GC and verify regularly to catch permission drift early