Data is written to tape one block at a time until all data is written or the tape becomes full. As data or images of backup jobs expire, the tape becomes fragmented with a mix of active data and expired data that is deleted from the TSM catalog or TSM database. Therefore, tapes that are marked full often contain less than 100% data.
The space reclamation feature frees space on tapes or sequential file volumes in sequential storage pools by moving all the data onto as few tapes as possible. However, because data from different streams from a parallel backup job might reside on one tape, multiple single-stream TARA jobs must run serially to complete the entire TARA restore job. This process degrades performance. Therefore, Teradata recommends disabling space reclamation.
Disabling space reclamation, however, has a side effect. As generations of backup images expire based on retention policy, the data or images marked for deletion accumulate on the tape. As long as there are some valid or active backup images on the tape, the tape cannot be cleaned or moved to scratch.