Data Guard synchronous (SYNC) vs asynchronous (ASYNC) modes

Oracle Database Tips by Donald BurlesonApril 20, 2015

Question: I am using data guard with asynchronous log transport mode ( ), and I am getting LNS wait on SENDREQ issues. What is the difference between synchronous (SYNC) mode and asynchronous (ASYNC) log transport modes? I have a very slow network with 25 milliseconds average packet shipment time. Which archive redo log transport mode is best, and why?

Answer: Oracle Data Guard redo log transport offers synchronous log transport mode (LogXptMode = 'SYNC') or asynchronous log transport mode (LogXptMode = 'ASYNC'). Systems with a network bottleneck would get better overall response time with the ASYNC mode, while shops that cannot tolerate a stale standby server (i.e. maximum availability or maximum protection are best with the SYNC mode.

The difference is all about when the COMMIT happens.

LogXptMode = ('SYNC'): As the name implies, SYNC mode synchronizes the primary with the standby database and all DML on the primary server will NOT be committed until the logs have been successfully transported to the standby servers. The synchronous log transport mode is required for the Maximum Protection and Maximum Availability data protection modes.
LogXptMode = ('ASYNC'): Conversely, asynchronous mode (ASYNC) allows updates (DML) to be committed on the primary server before the log file arrives on the standby servers. The asynchronous log transport mode is required for the Maximum Performance data protection mode.

Also see these important notes on LNS wait on SENDREQ.

The Oracle documentation notes these wait events for destinations configured with the LGWR SYNC Attributes

Wait Event	Monitors the Amount of Time Spent By . . .
LGWR wait on LNS	The LGWR process waiting to receive messages from the LNSn process.
LNS wait on ATTACH	All network servers to spawn an RFS connection.
LNS wait on SENDREQ	All network servers to write the received redo data to disk as well as open and close the remote archived redo log files.
LNS wait on DETACH	All network servers to delete an RFS connection.

Wait Events for Destinations Configured with the LGWR ASYNC Attributes

Wait Event	Monitors the Amount of Time Spent By . . .
LNS wait on DETACH	All network servers to delete an RFS connection.
LNS wait on ATTACH	All network servers to spawn an RFS connection.
LNS wait on SENDREQ	All network servers to write the received redo data to disk as well as open and close the remote archived redo log files.
True ASYNC Control FileTXN Wait	The LNSn process to get hold of the control file transaction during its lifetime.
True ASYNC Wait for ARCH log	The LNSn process waiting to see the archived redo log (if the LNSn process is archiving a current log file and the log is switched out).
Waiting for ASYNC dest activation	The LNSn process waiting for an inactive destination to become active.
True ASYNC log-end-of-file wait	The LNSn process waiting for the next bit of redo after it has reached the logical end of file.

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