There are two schools of thought concerning how replication is implemented. Many storage solutions use a snapshot-based approach. With snapshots, identifying the block change delta between two points in time is easy. However, as recovery point objectives (RPOs) get smaller, the number of required snapshots increases dramatically, which places hard limits on how small RPOs can be.
PowerFlex uses a journaling-based approach to replication. Journal-based replication provides the possibility of small RPOs, and, importantly, is not constrained by the maximum number of available snapshots in the system or on a given volume.
Checkpoints (or intervals) are maintained in journals, and the journals live as PowerFlex volumes in a storage pool in the same protection domain. However, the journal volume does not need to not reside in the same storage pool as the volume being replicated. The journal volumes resize dynamically as writes are committed, shipped, acknowledged, and deleted. So, the capacity that the journal buffer uses varies over time.