Calculate Proxmox tape retention from the media allocation policy

Work out the tape retention start event as well as its configured duration.

Source facts

Proxmox Backup Server tape retention begins at different points depending on media-set allocation. Continuing an existing media set starts its retention period when a newer set exists. Always creating a new set starts the clock at that set’s creation. Calendar-based allocation uses the next set’s creation or the next scheduled allocation event when no newer set exists. Large media sets also create restore dependencies across their tapes; even a single-VM restore can require data from all tapes in the set.

Source: Proxmox Backup Server — Tape Backup.

Applicability

This applies to Proxmox Backup Server’s documented tape media-set policies. It addresses the effective retention clock and restore dependency, not legal retention advice or a guaranteed restore duration.

DSE recommendation

DSE recommends writing worked calendar examples for the exact allocation and retention combination before assigning cartridges to the pool. Include a missed job or an off-site tape in that review so the team understands when reuse becomes possible. Keep media-set membership with the vault inventory, and have the recovery owner identify the complete set needed for a rehearsal. Avoid approving a policy solely from its retention-duration label without checking the event that starts that duration.

Verification

Using disposable media or a supported test setup, compare recorded set creation and allocation events with the calculated retention start. Rehearse locating the complete media set for a selected recovery point. Retain expected reuse dates and any missing-tape dependencies before adopting the production rotation.

Official reference

Proxmox Backup Server — Tape Backup. Reviewed October 15, 2026.