User`s guide

The next figure shows the actual usage of the tapes with free space instead of the deleted backups on

the first Friday of the following year. At the time the differential backup (blue rectangle) is written

onto tape 24.

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.