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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:


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