Seagate ST39175LW Instrukcja Naprawy Pobierz pdf (Strona 28)

Frozen Image Overview

14 NetBackup ServerFree Agent System Administrator’s Guide

Copy-on-write snapshot process

The immediate results of the snapshot are the following: a cached copy of those portions

of the source that were about to change at a certain moment (see step above), and a

record of where those cached portions (blocks) are stored ( ).

The copy-on-write snapshot does not produce a copy of the source; it creates cached

copies of the blocks that have changed and a record of their location. The backup process

refers to the source data or cached copy of the data as directed by the snapshot driver (see

figure “Backing up a snapshot-type frozen image” on page 15).

Image of source data is

frozen; snapshot driver is

activated

Snapshot cache

New write requests

(represented by arrows)

to s4, s7, s8 are held by

snapshot driver

s1 s2 s3 s4 s5 s6 s7 s8

s10

c1 c2 c3 c4

Source data, modified

after snapshot

Write requests are now

allowed to take place

s1 s2 s3 s4 s5 s6 s7 s8

s10

Record of cached blocks:

s1 s2 s3 s4 s5 s6 s7 s8

s10

Snapshot driver writes

contents of blocks s4,

s7, and s8 to cache

(“copy on write”)

Writes delayed

Snapshot driver keeps

a record of the writes to

cache

Source data

Result of

snapshot

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 ... 181 182

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Seagate ST39175LW Instrukcja Naprawy Strona 28