Asynchronous mode is useful when it is acceptable for the Secondary not to be up-to-date. When replicating in asynchronous mode, an update to the Primary volume is complete when it has been recorded in the Primary SRL. In asynchronous mode, all completed updates to the Primary volumes are guaranteed to be made on the Secondary data volumes with some delay. This is true despite failures in communication or system crashes on any of the participating hosts.
The application is informed that the write request is complete and the write is queued persistently to be sent to the Secondary. This queue may grow when there is a surge in the write rate. The queue is being continuously drained. When the surge subsides, the queue drains faster than it grows enabling the Secondary to catch up with the Primary. Because asynchronous mode queues writes persistently and holds them at the Primary for later transmission, it is able to deal with temporary outages of the network or the Secondary host without affecting the performance of the application. However, asynchronous mode has the disadvantage that if a disaster occurs, it is likely that the most recent writes have not reached the Secondary and therefore the data at a Secondary is not up-to-date when a failover occurs.
For more information about asynchronous mode, see "Asynchronous Mode Considerations" in the Veritas Volume Replicator Planning and Tuning Guide.