VERITAS File System 4.1 Administrator's Guide

Online Backup Using File System Snapshots
Snapshot File System Internals
Chapter 6124
Snapshot File System Internals
The following sections describe the internal structure of a snapshot file system and how it
copies changed data blocks from the original snapped file system.
Snapshot File System Disk Structure
A snapshot file system consists of:
• A super-block
• A bitmap
• A blockmap
• Data blocks copied from the snapped file system
The following figure shows the disk structure of a snapshot file system:
Figure 6-1 The Snapshot Disk Structure
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.
super-block
bitmap
blockmap
data block