VERITAS File SystemÖ 3.5 (HP OnlineJFS/JFS3.5) AdministratorÆs Guide (December 2002)

Chapter 5
Storage Checkpoints
How a Storage Checkpoint Works
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In the figure below, each block of the file system is represented by a square. Similar to the previous figure,
this figure shows a Storage Checkpoint containing pointers to the primary fileset at the time the Storage
Checkpoint is taken.
Figure 5-2 Initializing a Storage Checkpoint
The Storage Checkpoint presents the exact image of the file system by finding the data from the primary
fileset. As the primary fileset is updated, the original 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.
In the following figure, 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 5-3 Updates to the Primary Fileset
Primary Fileset
A
B
C
D
E
Storage Checkpoint
Primary Fileset
A
B
C’
D
E
Storage Checkpoint
C