Excessive lag between the Primary and the Secondary could be a liability in asynchronous replication. The Latency Protection feature of VVR protects the Secondary host from falling too far behind in updating its copy of data when replicating in asynchronous mode. This feature limits the number of outstanding writes lost in a disaster enabling automatic control of excessive lag between Primary and Secondary hosts when you replicate in asynchronous mode.
When replicating in asynchronous mode, it is normal for the SRL to have writes waiting to be sent to the Secondary. If your network has been sized based on the average update rate of the application on the Primary node, the number of writes waiting in the Primary SRL is likely to be within an acceptable range. The number of writes in the SRL would grow under the following circumstances:
If the Primary SRL has a large number of writes waiting to be transferred to the Secondary, the Secondary data is considerably behind the Primary. If a disaster strikes the Primary and the Secondary takes over, the Secondary would not contain all the data in the Primary SRL. In this case, the data on the Secondary would be consistent but significantly out of date when the Secondary takes over. To prevent the Secondary from being too far behind the Primary in this scenario, you can limit the number of writes waiting in the Primary SRL for transmission to the Secondary by setting up latency protection.