hadr_syncmode - HADR synchronization mode for log write in peer state configuration parameter

Configuration type

Database

Applies to

• Database server with local and remote clients
• Database server with local clients

Parameter type

Configurable

Default [range]

NEARSYNC [ASYNC, SUPERASYNC, SYNC]

Valid values for this parameter are:

SYNC

This mode provides the greatest protection against transaction loss, but at a higher cost of transaction response time.

In this mode, log writes are considered successful only when logs have been written to log files on the primary database and when the primary database has received acknowledgement from the standby database that the logs have also been written to log files on the standby database. The log data is guaranteed to be stored at both sites.

NEARSYNC

This mode provides somewhat less protection against transaction loss, in exchange for a shorter transaction response time than that of SYNC mode.

In this mode, log writes are considered successful only when the log records have been written to the log files on the primary database and when the primary database has received acknowledgement from the standby system that the logs have also been written to main memory on the standby system. Loss of data occurs only if both sites fail simultaneously and if the target site has not transferred to nonvolatile storage all of the log data that it has received.

ASYNC

Compared with the SYNC and NEARSYNC modes, the ASYNC mode results in shorter transaction response times but might cause greater transaction losses if the primary database fails.

In this mode, log writes are considered successful only when the log records have been written to the log files on the primary database and have been delivered to the TCP layer of the primary system's host machine. Because the primary system does not wait for acknowledgement from the standby system, transactions might be considered committed when they are still on their way to the standby.

SUPERASYNC

This mode has the shortest transaction response time but has also the highest probability of transaction losses if the primary system fails. This mode is useful when you do not want transactions to be blocked or experience elongated response times due to network interruptions or congestion.

In this mode, the HADR pair can never be in peer state or disconnected peer state. The log writes are considered successful only when the log records have been written to the log files on the primary database. Because the primary system does not wait for acknowledgement from the standby system, transactions might be considered committed when they are still on their way to the standby.

Figure 1 shows when the logs for transactions are considered successful based on the synchronization mode chosen:

Figure 1. Synchronization modes for high availability and disaster recovery (HADR)