Which applications would benefit from Oracle9i RAC and which ones would not?
If you believe the Oracle marketing hype, then all applications would benefit from RAC. But in truth, the ones that benefit the most are the ones that need either high availability or higher scalability, or both. If your application is running fine on a non-RAC implementation and you don't need the higher availability that a clustered database would provide, then I see no reason to make things much more complex by implementing RAC.
It used to be that applications had to be segmented to get good performance from RAC's predecessor, Oracle Parallel Server (OPS). OPS had a problem when an application on one node needed data in another node's buffer cache. The other node would have to write that data block to disk first before the first node could access it. This was known as "disk pinging." Towards the end of Oracle8i's life cycle, Oracle introduced a technology called Cache Fusion. Cache Fusion is one of the reasons that RAC is so successful for virtually any application. Cache Fusion eliminated disk pinging by letting Buffer Caches in each node move blocks back and forth across a high speed interconnect instead of disk. Now that Cache Fusion is in place, there really isn't any technological restrictions on applications that can successfully use RAC.
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