Figure 7.7 A shared memory SMP configuration.

There is a great deal of confusion about the difference between Oracle's parallel server and parallel query. While Oracle’s parallel query can be used with any computer configuration, including standalone processors, SMP, or MPP, Oracle’s parallel server can only be used on MPP systems. As stated earlier, MPPs are systems where a number of independent nodes, each with its own memory, share a common disk resource. As such, SMP is sometimes called shared memory multiprocessing and MPP is called shared-nothing multiprocessing. Examples of SMP processors include IBM SP2 and the IBM SP3, which contains eight processors. In parallel server parlance, a processor is called a node.

Oracle parallel server only works with MPP because each node on the MPP box requires its own memory area for the Oracle SGA. To illustrate, consider the example shown in Figure 7.6. Here, we see that a four processor system has been configured to share a common data resource. As such, any user, on any node, will get a complete view of the entire database. For example, user Scott could login to node 1 and create a public access table called TIGER. Immediately after creation, another user on node 4 could also access that table.

In parallel server, careful consideration must be given to the uses of parallelism, because the resulting system could often perform slower than a single node system. For example, in parallel server, the DLM may force the Oracle database writer to write transactions to the database more frequently than a standalone Oracle database.

When planning for parallel server tasks, it is a good idea to segregate specific types of tasks to specific nodes (see Figure 7.8). For example, common update routines against tablespace A could be segregated onto node 1, while queries against tablespace B could be segregated onto node 2. Because each Oracle instance has its own complete SGA, a full-table scan on one node will not flush any data out of the buffer pool of another node.