Veritas File System v5.0.1 Administrator's Guide for HP-UX 11i v3 (5900-0082, June 2010)

The Storage Checkpoint presents the exact image of the file system by finding
the data from the primary fileset. VxFS updates a Storage Checkpoint by using
the copy-on-write technique.
See “Copy-on-write” on page 87.
Copy-on-write
In Figure 6-3, the third data block in the primary fileset originally containing C
is updated.
Before the data 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 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. In this example, subsequent
updates to the third data 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