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
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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