Incremental backup chain restore blocked by missing parent backup in Ceph backend

Answer

No. A child incremental backup cannot be restored independently when its parent backup record or underlying Ceph objects are missing. The restore request fails and the target volume remains in an error state.

Confirmed facts

• Cinder stores the parent‑child relationship in its database.
• The restore API checks for the parent backup record; if it cannot be found it raises a BackupNotFound exception.
• The cinder‑backup service logs an ERROR line indicating the missing parent.

Likely explanation (version‑dependent)

The exact log text varies between releases. In older releases (Stein/Ussuri) you typically see:

ERROR cinder.backup.driver [req-...] Backup  not found

Starting with Wallaby/Zed the message includes both IDs:

ERROR cinder.backup.driver [req-...] Failed to locate parent backup  for incremental backup 

Both variants are translated from a librados ObjectNotFound error when the driver tries to read the parent RBD snapshot.

Steps to verify the failure mode

1. Take a full backup, then an incremental backup of a volume.
2. Delete the full backup: cinder backup-delete .
3. Attempt to restore the incremental backup: cinder restore .
4. Check the cinder‑backup log (usually /var/log/cinder/cinder-backup.log) for an ERROR line containing either “Backup not found” or “Failed to locate parent backup”.
5. Confirm the volume status is error or error_restoring with cinder show .

Supported workflow to recover

There is no supported Cinder‑only method to reconstruct or bypass a missing parent link. The only reliable way to restore the child is to have the parent backup available (either restored from an external copy or recreated). Manually injecting the parent RBD objects outside of Cinder is possible but unsupported and may break consistency guarantees.

Behavior across other incremental‑capable drivers

• NFS driver: logs a “Parent backup file not found” error when the parent file is missing.
• S3 driver: logs an S3 NoSuchKey error for the missing parent object.
• All drivers require the parent backup to be present for an incremental restore, so the failure mode (missing parent → restore abort) is consistent across Ceph/RBD, NFS, and S3 backends.

Missing diagnostic detail

To give you the exact log message you should expect, please confirm which Cinder release (e.g., Stein, Ussuri, Wallaby, Zed, etc.) you are running.