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The full backup stored on tape 01 is deleted after the next full backup is created onto both tapes 23
and 24 on Friday of the 52nd week. As all backups of tape 01 have been deleted, the tape is
considered as free and can be reused.
Further analysis of the example proves that the maximal number of tapes required to store the data
backups is 25 tapes. This maximum occurs on the 16th week of the following year.
The above mentioned figures show that a data recovery requires one or two tapes for a full backup,
two or three tapes for a differential backup, and one, two or three tapes for an incremental backup.
For example, if we need to recover data from a backup created on Monday of the 52nd week, the
task will require the following tapes:
Tape 23 with an incremental backup (marked with "52") and a differential backup created on
Friday of the 51st week
Tape 21 and Tape 22 that contain a full backup created on Friday of the 48th week.
The example reveals the following shortcomings of the scheme combination with the specified tape
options:
commonly any data recovery is a long process that requires loading, mounting, rewinding and
reading of one (3% - for backups displayed in the "Tape usage during the first year" figure), two
(65%) or three (32%) tapes
22 tapes are used to store 13 monthly full backups when the monthly backup size is less than the
size of a tape, so keeping data is more expensive
25 tapes are required for full year rotation of the data backups.
GFS Example 2
Suppose, 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 cleared
•
the
Always use a free tape: For each differential backup
option is cleared.
The example has only one difference from the previous one. That is selection of the
Always use a
free tape: For each full backup
option.
Below, the figure shows the deleted backups as actual, but demonstrates tape usage during the
whole year for the GFS backup scheme in combination with the specified tape options. A number in
the green rectangle marks an incremental backup created on Monday of the corresponding week of
the year.
