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Frozen Image Overview

12 NetBackup ServerFree Agent System Administrator’s Guide

Frozen Image Overview

Large active databases or file systems that must be available around-the-clock are difficult

to back up without incurring a penalty. Often, the penalty takes one of two forms:

◆ The entire database is taken offline or the file system is unmounted, to allow time for

the backup, resulting in suspension of service and inconvenience to users.

◆ The copy is made very quickly but produces an incomplete version of the data, some

transactions having failed to complete.

A solution to this problem is to create a frozen image of the data. This means “capturing”

the data at a particular instant and making a copy of it, without causing significant

downtime. The resulting copy or image can then be backed up without affecting the

performance or availability of the file system or database application data. Without a

complete, up-to-date image of the data, a correct backup cannot be obtained.

When a backup is managed by a backup agent on a Fibre Channel network, the data to

back up must be contained in a frozen image. The backup agent can only access the data

by means of the raw physical disk. Once the data is captured as a frozen image, the

NetBackup client “maps” the logical representation of the data to its absolute physical

disk address. These disk addresses are sent to the backup agent over the LAN and the

data is then read from the appropriate disk by the backup agent. (This process is

explained in greater detail under “Offhost Backup Overview” later in this chapter.)

Two types of frozen image methods are available, both supported by NetBackup:

◆ copy-on-write snapshots

◆ mirrors

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