Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

Figure 18-1
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.
Differences between snapshots and Storage
Checkpoints
While snapshots and Storage Checkpoints both create a point-in-time image of a
file system and only the changed data blocks are updated, there are significant
differences between the two technologies:
Table 18-1
Differences between snapshots and Storage Checkpoints
Storage CheckpointsSnapshots
Reside on the same device as the original file
system
Require a separate device for storage
Can be read-only or read-writeAre read-only
263Administering snapshot file systems
Differences between snapshots and Storage Checkpoints