VVR replicates in synchronous and asynchronous modes. The decision to use synchronous or asynchronous mode must be made with an understanding of the effects of this choice on the replication process and the application performance. Replication modes are describe in more detail in Modes of replication.
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. Asynchronous mode does not guarantee the data is current at all times, but it has less impact on application performance and provides the ability to use more cost-effective telecommunications. All completed updates to the Primary volumes are guaranteed to be made on the Secondary data volumes with some delay.
Synchronous mode ensures that a write has been posted to the Secondary and the Primary before the write completes at the application level. When replicating in synchronous mode, the data on the Secondary is completely up-to-date and if a disaster occurs at the Primary, data can be recovered from any surviving Secondary without any loss. If the Secondary must reflect all writes that have successfully completed on the Primary, synchronous mode is the correct choice.
Synchronous replication provides data currency but can impact application performance in high latency or limited bandwidth environments. The response time experienced by the application is affected because the write has to wait for the Secondary to acknowledge it before the write can complete on the Primary.