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.

Acronis Backup & Recovery 1149 GFS Example 3

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.

GFS Example 3