CHAPTER 7 n CLUSTERING 184
Active/Active
The active/active configuration is by far the most common implementation of Windows
clustering that I??™ve encountered. Even though there is no double licensing cost for SQL
Server in an active/passive environment, management is rarely comfortable with the
thought of a hardware investment that ???does nothing??? (apparently providing redundancy
has no value). Thus, both servers are used, each hosting its own SQL Server instance.
Running in active/active mode adds complexity to the configuration of each individual
node, as well as to the individual hardware requirements, as you can see in Figure 7-3.
Each node must have sufficient hardware to run both instances of SQL Server. You also
must configure that hardware (by setting the maximum memory, CPU affinity, and so on)
so that in the event of a failover, the second SQL Server instance won??™t collide with the
first. Most clients I??™ve worked with will accept a certain level of performance degradation,
but keep in mind that a hardware failure might last for a period of days or weeks while
the replacement equipment is being acquired and configured.
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