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Frozen Image Overview

14 NetBackup ServerFree Agent System Administrator’s Guide

As shown in Figure 4, an accurate backup is obtained by combining the unchanged

portions of the source with the snapshot cache. When a backup of the snapshot frozen

image begins, the backup application copies the source data until it comes to a block

that changed after the snapshot driver was activated. The snapshot driver tells the backup

process to skip that changed block and read in its place the cached (original) copy . The

backup application continues copying source data until it comes to another changed

block. Cache is read again as the snapshot driver dictates. The backup, when finished,

is an exact copy of the source as it existed the moment the snapshot driver was activated.

In NetBackup ServerFree Agent, the nbu_snap and fsclone frozen image methods

provide support for copy-on-write snapshot frozen images.

Mirror

Unlike a copy-on-write snapshot, a mirror is a complete copy stored on a separate disk,

physically independent of the source. Every change or write to the source data on the

primary disk is also made to the copy on the secondary disk. This creates a “mirror”

image of the source.

Figure 5. Source and mirror images

As in a copy-on-write snapshot, transactions are allowed to finish and new I/O on the

source is briefly halted. When the mirror image is brought up-to-date with the source

(made identical to it), changes to the source can continue and the mirror image can be

backed up. Since mirroring requires an exact, complete copy of the source on a separate

device (equal in size to the disk being mirrored), it consumes more disk space than a

copy-on-write snapshot.

In NetBackup ServerFree Agent, the vxvm frozen image method provides support for

mirror frozen images.

All data written to source

is also written to mirror

Source image

(primary)

Mirror image

(secondary)

NetBackup 4.0V ServerFree Agent System Administrator's Guide for Solaris

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Seagate ST39175LW Guía de usuario Pagina 24