Taper scan algorithm

The Taper scan algorithm is the algorithm used to determine which volume, if any, is the next to be written by taper. The policy is as follows:

Consider the set of tapes T. We can partition the set into two disjoint subsets A (the set of active tapes) and I (the set of inactive tapes). Assuming I is nonempty, there exists a subset P of I, called the set of preferred tapes. Note that T = A + I, P ⊆ I, and A ∩ I = ∅.

Amanda will only use tapes from I; active tapes are not considered for overwriting. Also, tapes from P are preferred to other tapes in I; a tape not in P (but in I) will be used only if no tapes in P are available. If no tapes from I are available, then no tapes are used and Amanda will go into degraded mode.

Tapes are assigned to each of the sets as follows:

• Any labeled but unused tapes are in both I and P. This includes unlabeled tapes if the label_new_tapes option is set.
• The most recently used "tapecycle" number of tapes is in A.
• Any remaining tapes are in I. The single least recently used of these is also in P.

This algorithm is applied from scratch any time a new tape is needed during a backup run. You can run the algorithm without running Amanda by doing 'amtape taper'.