NEARSYNC mode

NEARSYNC mode is comparable to the SYNC, mode at significantly less cost. Standby sends ack message as soon as it receives the logs in memory. It also sends logs to standby and writes logs to primary disk in parallel. On a fast network, log replication causes no or little overhead to primary log writing. In NEARSYNC mode, you lose data if primary fails and the standby fails before it has a chance to write the received logs to disk. This is a rare "double failure" scenario. Thus NEARSYNC mode is a good choice for many applications, providing near synchronization protection at a far less performance cost.

Parent topic: Description of the HADR Modes