![]() ![]() Network transmission delay does not impact performance in this mode. ASYNC mode is well suited for WAN application. Because ASYNC mode does not wait for ack messages from the standby, primary system throughput is min(log write rate, log send rate). In ASYNC mode, sending logs to standby and writing logs to primary disk are done in parallel, just like NEARSYNC mode. ![]() Thus NEARSYNC is a good choice for many applications, providing near synch protection at far less performance cost. This is a relatively rare "double failure" scenario. In NEARSYNC mode, you will lose data if primary fails and the standby fails before it has a chance to write received logs to disk. On a fast network, log replication causes no or little overhead to primary log writing. Furthermore, sending logs to standby and writing logs to primary disk are done in parallel. Standby sends ack message as soon as it receives the logs in memory. NEARSYNC mode is nearly as good as SYNC, at significantly less cost. The cost of replication is significantly higher than that of other modes. The total time for a log write is the sum of (primary_log_write + log_send + standby_log_write + ack_message). Log write and replication events happen sequentially. In SYNC mode, logs are sent to standby only after they are written to primary disk. ![]() The cost is the extra time for writing on standby and sending the ack message back to primary. Two on-disk copies of data are required for transaction commit. ASYNC and SUPERASYNC modes are typically used over WAN. SYNC and NEARSYNC modes are typically used on LAN. For SUPERASYNC mode, primary log writing is independent of log replication. For ASYNC mode, primary will consider replication done as soon as the logs are delivered to the TCP layer of the primary host machine. SUPERASYNC - Transactions on primary does not wait for replication of logs to the standby.įor SYNC and NEARSYNC modes, the primary will wait for an ack message from the standby to confirm that the logs have been received and written to disk on standby (SYNC mode) or have been received on the standby (NEARSYNC mode).ASYNC - Transactions on primary will commit only after relevant logs have been written to local disk and sent to standby.NEARSYNC - Transactions on primary will commit only after relevant logs have been written to disk on primary and received into memory on standby.SYNC - Transactions on primary will commit only after relevant logs have been written to disk on both primary and standby.Database configuration parameter hadr_syncmode can be set to one of SYNC, NEARSYNC, ASYNC, or SUPERASYNC. HADR provides 4 synchronization modes to suit a diverse range of operational environment. See also High availability disaster recovery (HADR) synchronization mode HADR synchronization mode is controlled by the database configuration parameter hadr_syncmode. ![]()
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