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To simplify the tape planning lets discard the possibility that the calculated tapes might contain
backups of other data. It is implied that the Use a separate tape set option is enabled.
To calculate the number of tapes you should take into account the following considerations:
full backup size
average size of incremental backups
average size of differential backups
compression level specified for backing up the data
tape rotation scheme ( frequency of backups, retention rules)
tape-append options
requirements to support off-site tape cartridge archives.
There is no common formula to calculate a number of tapes required in all possible combinations of
above listed considerations. But the general way to get a number of tapes for a case includes the
following steps:
1.
Draw (or write) a chain of backups until the first backup can be deleted
2.
Take into account the tape-append options, the chain might be sectioned onto tape sets
3.
Calculate the number of tapes in each tape set
4.
The sum of the calculated values gives the total number of tapes required for the case.
Tape planning: Example 1
Imagine the case with the following features:
full backup size is F_GB
average size of incremental backups is I_GB
average size of differential backups is D_GB
compression level provides CL average reduction coefficient
selected tape rotation scheme is Tower of Hanoi with four levels
tape options are the following:
the Use a separate tape set option is selected
the Always use a free tape: For each full backup option is cleared
the Always use a free tape: For each incremental backup option is cleared
the Always use a free tape: For each differential backup option is cleared
tape size is T_GB.
The Tower of Hanoi scheme with four levels (A, B, C, and D) specifies the following line of backups on
the tapes before the first backup will be deleted: D (full), A, B, A, C, A, B, A, D, A, B, A, C. The specified
tape options do not require using a free tape for any backup, so the backup line will be automatically
split and continued on a new tape when the end of the current tape is reached. There is one tape set
to calculate.
Total number of required tapes = round up ((2*F_GB + 6*I_GB + 5*D_GB) * CL / T_GB) + 1.
The above described ToH Example 1 (p. 161) is based on the five-level Tower of Hanoi backup
scheme with the same tape options. Its backup line was the following: E (full), A, B, A, C, A, B, A, D, A,
B, A, C, A, B, A, E, A, B, A, C, A, B, A, D.
