In Figure: Updates to the primary fileset, 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.