Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Online Backup Using File System Snapshots
Snapshot File System Internals
Chapter 6108
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