This figure shows a test master data file with its data blocks, a snapshot data file pointing to the test master's data blocks, and another snapshot data file pointing to the first snapshot’s data blocks. The first snapshot data file points to all the same data blocks as the test master’s except for one data block. This one data block was allocated on-write for the snapshot data file. The second snapshot data file points to all the same data blocks as the first snapshot’s, except for one data block.