Veritas™ File System 5.0.1 Administrator's Guide

Figure 5-1

The Snapshot Disk Structure

super-block

bitmap

blockmap

data block

The super-block is similar to the super-block of a standard VxFS file system, but

the magic number is different and many of the fields are not applicable.

The bitmap contains one bit for every block on the snapped file system. Initially,

all bitmap entries are zero. A set bit indicates that the appropriate block was

copied from the snapped file system to the snapshot. In this case, the appropriate

position in the blockmap references the copied block.

The blockmap contains one entry for each block on the snapped file system.

Initially, all entries are zero. When a block is copied from the snapped file system

to the snapshot, the appropriate entry in the blockmap is changed to contain the

block number on the snapshot file system that holds the data from the snapped

file system.

The data blocks are filled by data copied from the snapped file system, starting

from the beginning of the data block area.

How a snapshot file system works

A snapshot file system is created by mounting an empty disk slice as a snapshot

of a currently mounted file system. The bitmap, blockmap and super-block are

initialized and then the currently mounted file system is frozen. After the file

system to be snapped is frozen, the snapshot is enabled and mounted and the

snapped file system is thawed. The snapshot appears as an exact image of the

snapped file system at the time the snapshot was made.

See “Freezing and thawing a file system” on page 73.

Initially, the snapshot file system satisfies read requests by finding the data on

the snapped file system and returning it to the requesting process. When an inode

update or a write changes the data in block n of the snapped file system, the old

data is first read and copied to the snapshot before the snapped file system is

updated. The bitmap entry for block n is changed from 0 to 1, indicating that the

Online backup using file system snapshots

How a snapshot file system works