Description of the HADR Modes
HADR synchronization mode is controlled by the database configuration parameter hadr_syncmode. See High Availability Disaster Recovery (HADR) synchronization mode.
SYNC Transactions on primary commits only after relevant logs are written to disk on both primary and standby. NEARSYNC Transactions on primary commits only after relevant logs are written to disk on primary and received into memory on standby. ASYNC Transactions on primary commits only after relevant logs have been written to local disk and sent to standby. SUPERASYNC Transactions on primary does not wait for replication of logs to the standby.
For SYNC and NEARSYNC modes, the primary waits for an ack message from the standby to confirm logs are received and written to disk on standby (SYNC mode) or are received on the standby (NEARSYNC mode). For ASYNC mode, primary considers replication as done as soon as the logs are delivered to the TCP layer of the primary host machine. For SUPERASYNC mode, the primary log writing is independent of log replication. SYNC and NEARSYNC modes are typically used on LAN. ASYNC and SUPERASYNC modes are typically used over WAN.
See:
Parent topic: DB2 HVDB High Availability Disaster Recovery (HADR) guideline