Modes of Replication

VVR replicates in synchronous and asynchronous modes. You can set the mode of replication between the Primary and each Secondary depending on your requirement. You can replicate to the Secondary hosts of an RDS in different replication modes.

In each mode, VVR replicates and completes the application writes differently. Each mode deals differently with network conditions. The following sections provide a brief description of synchronous and asynchronous modes of replication and discuss some of the issues involved in choosing between the two. For more information, see the VERITAS Volume Replicator Planning and Tuning Guide.

Asynchronous 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. 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. If a disaster strikes in asynchronous mode, it is likely that the most recent writes have not reached the Secondary. Asynchronous mode has the disadvantage that the data at a Secondary might not have the latest updates when a failover occurs. For more information about asynchronous mode, see "Asynchronous Mode Considerations" in the VERITAS Volume Replicator Planning and Tuning Guide.

Synchronous Replication

Synchronous mode ensures that a write has been posted to the Secondary and the Primary before the write completes at the application level. 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 keeps the Secondary up-to-date with the Primary by waiting for each write to reach the Secondary before the application sees the successful completion of the write on the Primary.

Synchronous replication provides data currency but can impact application performance in high latency or limited bandwidth environments. When using synchronous replication, 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.

Synchronous replication is most effective in application environments with low update rates. However, it can be deployed in write-intensive environments where high bandwidth, low latency network connections are available.

The performance of synchronous replication could degrade significantly if the peak application write rate exceeds the available network bandwidth. The amount of degradation can be reduced by increasing network bandwidth and reducing network latency between the Primary and Secondary. Refer to the VERITAS Volume Replicator Planning and Tuning Guide for a discussion of network latency and network bandwidth, and their effects on VVR performance.

In synchronous mode, the synchronous attribute enables you to specify what action is taken when the Secondary is unreachable. The synchronous attribute can be set to override or fail. When the synchronous attribute is set to override, synchronous mode converts to asynchronous during a temporary outage. In this case, after the outage passes and the Secondary catches up, replication reverts to synchronous.

When the synchronous attribute is set to fail, the application receives a failure for writes issued while the Secondary is unreachable. The application is likely to fail or become unavailable, and hence this setting must be chosen only if such a failure is preferable to the Secondary being out of date.

We recommend setting the synchronous attribute to override, as this behavior is suitable for most applications. Setting the synchronous attribute to fail is suitable only for a special class of applications that cannot have even a single write difference between the Primary and Secondary data volumes. In other words, this mode of operation must be used only if you want an application write to fail if the write cannot be replicated immediately. It is imperative that the network connection between hosts using this option must be highly reliable to avert unnecessary application downtime as network outage could cause an application outage. For more information about synchronous mode, see "Synchronous Mode Considerations" in the VERITAS Volume Replicator Planning and Tuning Guide.