Seagate ST39175LW Guía para resolver problemas descargar pdf (Pagina 27)

Chapter 1, Introduction 13

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 injected into the host operating system. 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 “Copy-on-write snapshot process” on page 14.)

1 2 ... 22 23 24 25 26 27 28 29 30 31 32 ... 181 182

Sin comentarios

Seagate ST39175LW Guía para resolver problemas Pagina 27