VERITAS File System 3.5 (HP OnlineJFS/JFS 3.5) Administrator's Guide (August 2003)

Chapter 5
Online Backup
Snapshot File System Internals
69
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 5-1 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.
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.
super-block
bitmap
blockmap
data block