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Don Burleson Blog 







Failover between two servers

Oracle Database Tips by Donald BurlesonAugust 5, 2016

Question: I want to use two servers to failover to each other and I want to size the servers such that the databases will work when a failover occurs.  What parameters do I need to set and how do I manage a back-and-forth database failover.  I am using Data Guard physical standby.

Answer:  The physical standby database requires that both instances already exist on both servers, so the SGA RAM is already allocated in case if failover.  If one of the servers failed and one of the failover databases became a primary, you would see a spike in CPU usage as new connections were directed to the new instance.  You might also see a PGA RAM shortage which manifests itself as a spike in disk sorts and a decrease in hash joins.

The most common error when DBA's set up a failover server that is also an active database is that they forget to set the OS parameter MAXUPROC to allow for the increase in system processes.  (This advice only applies to system that use dedicated connections, and it does not apply to system with connection pooling (shared servers, MTS)).

Normally, a DBA will make allowances for enough CPU to support two instances and you can check the CPU runqueue values as compared to the cpu_count.  Remember, a failover condition is rare and it does not make sense to deliberately over-allocate server resources for an event that is not likely to happen.

Today's servers have a Mean Time Between Failures (MTBF) that is expressed in decades and a Mean Time To Recovery (MTTR) that is expressed in hours, so the tradeoff is such that a few hours of slow response time every ten years is better than spending thousands of dollars in extra CPU's and will not be used.

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