HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

84 Chapter4
Online Backup
Snapshot File Systems
Figure 4-1 The Snapshot Disk Structure
The super-block is similar to the super-block ofa normal VxFS file
system, however, the magic number is different and many of the fields
are meaningless.
Immediately following the super-block is the bitmap. 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 will reference the copied block,
Following the bitmap is the blockmap. It contains one entryfor 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 used by the snapshot file system are located after the
blockmap. These are filled by any data copied from the snapped file
system, starting from the front 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 (see “Freeze and Thaw”, for a description of the VX_FREEZE ioctl).
Once 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
Blockmap
Data Blocks
Super-Block
Bitmap