GFS Example 3

Imagine the backup plan has the following tape options:

• the Use a separate tape set option is selected
• the Always use a free tape: For each full backup option is selected
• the Always use a free tape: For each incremental backup option is selected
• the Always use a free tape: For each differential backup option is selected.

These options define the tape rotation scheme that is classical for GFS. The figure shows the beginning of the rotation scheme that uses 8 tapes for daily backups, 6 tapes for weekly backups and 13 tapes for monthly backups (since there are 13 four-week cycles in a year) for the analyzed case. And one tape is required for the next backup. In total this rotation scheme, combined with the options requires 28 tapes.

To recover the data only one tape is required for a full backup, two tapes for a differential backup, and two or three tapes for an incremental backup.

This scheme has the following advantages:

• access to any full backup requires only one tape
• backup deletion frees a tape so it can be reused.

The main drawback is the large number of required tapes that is used 5-10%.

If we have to keep a daily backup for a week (4 backups) and a weekly backup for a month (4 backups), the total number of required tapes will be equal to 4+4+13+1 = 22.