Veritas™ File System 5.0.1 Administrator's Guide

data is copied to the Storage Checkpoint before the new data is written. When a
write operation changes a specific data block in the primary fileset, the old data
is first read and copied to the Storage Checkpoint before the primary fileset is
updated. Subsequent writes to the specified data block on the primary fileset do
not result in additional updates to the Storage Checkpoint because the old data
needs to be saved only once. As blocks in the primary fileset continue to change,
the Storage Checkpoint accumulates the original data blocks.
Copy-on-write
In Figure 6-3, the third block originally containing C is updated.
Before the block is updated with new data, the original data is copied to the Storage
Checkpoint. This is called the copy-on-write technique, which allows the Storage
Checkpoint to preserve the image of the primary fileset when the Storage
Checkpoint is taken.
Every update or write operation does not necessarily result in the process of
copying data to the Storage Checkpoint. In this example, subsequent updates to
this block, now containing C', are not copied to the Storage Checkpoint because
the original image of the block containing C is already saved.
Figure 6-3
Updates to the primary fileset
A
B
C’
D
E
C
Primary fileset Storage Checkpoint
87Storage Checkpoints
How a Storage Checkpoint works