How Long Is the Primary/Standby Replication Delay?

If the replication thread is running properly, replication delay is a value measured in seconds. Replication delay is NULL if the SQL thread is abnormal, or if the I/O thread is running normally but the SQL thread has applied all relay logs.

The synchronization delay between primary and standby instances cannot be calculated using a formula and is affected by the following factors:

• Network communication conditions.
• Transaction pressure on the primary instance, specifically the number of transactions executed per second on the primary instance.
• Transaction size on the primary instance, specifically how long a single transaction takes to execute.
• Load on the standby instance and read replicas.
• If the recovery model is set to Simple, the primary/standby relationship cannot be established and the maximum value of the Replication Delay metric will be reported.

If the primary DB instance has a heavy load for a certain period of time and executes a large number of transactions per second, replication to the standby DB instance will be delayed. This delay is generally a few seconds.

RDS for SQL Server: To check data consistency between the primary and standby DB instances, view Replication Delay on the Cloud Eye console to obtain the value of the primary/standby replication delay.