Synchronous Replication proxy's replication data like Server IO to a volume. Every write issued by the server is sent to the local volume and to the replication. The Compellent does not acknowledge completion of the write back to the Server until both the write IO to the local volume and the IO sent over the replication have completed. This means the local volume and replication are fully synchronized. There will be no data loss in the event of a failure on the local system.
Synchronous replication does not maintain recovery points between the local volume and the replication. This means that if connectivity is lost between the Replicating System and the Target System, the entire data volume must be re-copied to ensure all data is present and accounted for in both locations.
Asynchronous Replication acknowledges a write IO back to the server as soon as it has been completed on the local system. The write IO is also queued for delivery to the Target System. This allows for more efficient link utilization and data optimization on the link. It also means that writes present on the local system may not be present on the Target System in the event of a local failure -- you may lose writes.
Asynchronous Replication also uses the Data Instant Replay feature to create checkpoints between the local volume and the replication. These checkpoints serve as synchronization points to reduce the amount of data that needs to be transferred from the local system to the replication in the event of a communication failure between the Replicating System and the Target System.
You must choose either Synchronous or Asynchronous replication.
QoS definitions define the link attributes and bandwidth limiting for replications using that link. Specify a QoS definition as part of a replication. If you do not have a QoS definition, click on Create QoS Definition to create one.