Example

Example

To make things a bit clearer, let's look at an example. We have a machine with 20 GB of data, and each day about 5% of this data changes. This means we will have to back up about 1 GB of data for each incremental backup. The network will be the determining factor for the data transfer rate (we will assume a 16 Mbps token-ring network), and backup and restore throughput is equal. Table 1 shows times needed for backup operations and the type of backups:

Pattern		Sun	Mon	Tue	Wed	Thu	Fri	Sat
Full/Incremental	Type	Full	Incr	Incr	Incr	Incr	Incr	Incr
Full/Incremental	Time (sec)	5000	250	250	250	250	250	250
Full/Differential	Type	Full	Incr	Incr	Diff	Incr	Incr	Diff
Full/Differential	Time (sec)	5000	250	250	750	250	250	750
Incremental	Type	Incr	Incr	Incr	Incr	Incr	Incr	Incr
Incremental	Time (sec)	250*	250	250	250	250	250	250

* The first incremental backup will take 5000 seconds but here we assume that Sunday's backup is not the first backup.

Table 1. Backup Operation: Time Required Using Specific Backup Patterns

If we look at the restore operation, we will need to determine the number of tapes that are required and the time needed for restoring the data. Let's assume that we have to do a full restore (that is, 20 MB) on Friday (restoring from Thursday's backups).

Type	Number of tapes	Time (seconds)
Full/Incremental	5 Sun, Mon, Tue, Wed, Thu	6000 (5000 + 4x250 = 6000)
Full/Differential	3 Sun, Wed, Thu	6000 (5000 + 750 + 250 = 6000)
Incremental	Unknown	5000

Table 2. Restore Operation: Total Number of Tapes and Total Amount of Time Required

From this we conclude:

•  A full restore is faster when using incremental strategies, but the number of  tapes needed is hard to predict.
•  The number of tapes is the least when using the differential pattern.

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