A summery about VVR replication mode.

By my understanding the main difference between Sync and Async for VVR is how VVR writes VVR log and ship it to DR side. [@more@]

Async mode,

1> Oracle messages OS to request IO operations to write oracle redo buffer into a certain group of log files when a client process commits a transaction.

2> Oracle writes redo buffer into a certain group of log files.

3> Oracle completes the transaction and won’t wait for VVR.

4> VVR get the message from OS immediately and writes the log entries about Changed OS blocks into local VVR log and then ship the VVR log to DR side immediately.

5> VVR applies VVR logs transferred from primary side catch up with the primary side at OS level – OS blocks synchronize, which doesn’t affect the performance at primary side at all.

6> The potential data loss is the delay of VVR log transfer over LAN when the primary crash.

Sync mode,

1> Oracle messages OS to request IO operations to write oracle redo buffer into a certain group of log files when a client process commits a transaction.

2> Oracle writes redo buffer into a certain group of log files while VVR writes VVR log at remote side over LAN and applies the log to catch up the primary.

3> Oracle can’t finish the transaction until VVR writes VVR log at remote side over LAN and applies the log to catch up the primary as well as for sure Oracle writes redo buffer into a certain group of log files.

4> There will be no potential data loss after failover to DR.