Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 4 PLANNING
Disk Speed
Pay attention to your disk ratings because disk I/O can be a big performance bottleneck.
If it takes 10 milliseconds to retrieve one data block (typically 8 KB in size), then an SQL
query may not be able to read more than 100 data rows per second, because the required
rows will more likely than not be spread out over as many data blocks.
Memory Sizing
Memory sizing is heavily dependent on the characteristics of your application. The amount
of space you need in order to meet performance targets is very difficult to estimate unless
you have good information, such as the results of volume testing. The cost of memory is a
consideration, but memory becomes cheaper every year and it is no longer unusual to find
databases using many gigabytes of memory. Here are the main considerations when planning
memory requirements:
Buffer Cache: You need to allow enough space for Oracle to cache data blocks in
memory because disk I/O can slow down your application tremendously. Note that
you must a use a 64-bit operating system and the 64-bit version of Oracle in order
to create very large buffer caches because the 32-bit versions cannot handle large
amounts of memory.
Shared Pool: The shared pool contains the library cache and the dictionary cache.
Query execution plans are stored in the library cache, and information about database
objects is stored in the dictionary cache. It is rare to find a database that needs
more than 1 GB of memory for the shared pool.
Stack Space: You need to provide stack space for each Oracle connection. A commonly
used rule of thumb is 512 KB for each connection, for example, 50 MB for
100 connections.
CPU Sizing
The number of CPUs you need depends not only on the speed of the CPUs—the faster the
CPUs, the fewer you need—but also on the application load. A good rule of thumb is as
many CPUs as the average number of simultaneously active sessions during the critical
period of the day; the database for a big e-commerce application may require 16 CPUs to
meet performance targets.
Note that if you use more than four CPUs, you will need to purchase Oracle Enterprise
Edition, which is far costlier than Oracle Standard Edition.