Seagate ST39175LW Podręcznik Użytkownika Pobierz pdf (Strona 21)

Chapter 1, Introduction 11

Frozen Image Overview

Copy-on-Write Snapshot

A copy-on-write snapshot is a detailed account of data as it existed at a certain moment.

Unlike a mirror, explained in the next section, a copy-on-write snapshot is not really a copy

of the data, but a particular “record” of it.

The copy-on-write snapshot process works as follows: when a frozen image is required,

any unfinished transactions or changes to the source data are allowed to complete, but

new changes are temporarily stalled. The source is momentarily idled (made quiescent),

and a snapshot driver is activated. Once the snapshot driver is activated, new transactions

or changes (writes) to the source data are allowed to take place. However, the snapshot

driver briefly intercepts or holds the write requests. While holding those requests, it

copies to cache any blocks that will be affected by those writes, and keeps a record of the

cached blocks. In other words, it reads each source block that is about to change, copies

the block’s current data to cache, and records the location and identity of the cached

blocks. Then the intercepted writes are allowed to take place in the source blocks. (See

Figure 3.)

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Seagate ST39175LW Podręcznik Użytkownika Strona 21