VVR is configured such that the volumes to be replicated for a specific application are placed in an RVG. Writes to the data volumes are persistently queued in the SRL. The SRL on the Primary tracks all writes in the order in which they were received and transmits the writes to the Secondary using a replication link (RLINK).
On the Primary, each write to an RVG generates two writes: one to a data volume, and one to the SRL. While VVR generates two writes, only the write to the SRL affects the application. The write to the SRL is a fast write to a sequentially accessed log while the data volume write is a normal write performed asynchronously. The write to the data volume is not in the critical path for the application. The write to the data volume is written in the background and does not affect application performance.
If the Primary crashes at any point after the write completes to the SRL and before the write completes to the data volume, data is fully recoverable from the SRL. This is very similar to a database writing to a redo log and later writing to the data files. This two phase write gives VVR the ability to maintain write-order fidelity at the Secondary.