The Pointer-based Snapshot is a space optimal solution i.e it does not require the same size of source volume. But it requires a small fraction of the size of the source volume and this space is referred as save location. When a write is issued to the source volume for the first time, the original data in the source volume at that location is copied to this save location. So, when a new snapshot is created, it should support 8 hours data change which is 20 GB (save location size).
Please refer the topic “Pointer-based Virtual Replication – Snapshot” in Module 14-Replication.