For a number of years I have been deeply involved with clusters, both of the Linux persuasion and other kinds (see openMosix). I have always been interested in application-layer based clustering products: That is, application software that has clustering built-in. Oracle has offered a clustering product for quite some time; Oracle Parallel Server did, in fact, find a niche market for itself and, if used appropriately for its capabilities (read-only distributed access to databases), it fared quite well.

In July 2001—a year ago—Oracle introduced the Real Application Cluster (RAC) option for Oracle9i. RAC aims to exceed the capabilities of the former Parallel Server in the areas of scalability and high availability. To this end, Oracle adapted their Cache Fusion technology for inclusion in RAC. Cache Fusion guarantees cache coherency among multiple cluster nodes (separate Linux boxes running a similarly configured RAC instance) accessing the same databases, without incurring high I/O costs. As I have written many times in previous columns here at Byte.com, you can't use NFS or other similar network file systems for shared network file access for databases, because they don't guarantee a coherent locking mechanism and their protocols do not provide for cache coherency. Cache Fusion, on the other hand, was designed with these capabilities from the ground up (as was the openMosix File System in openMosix).

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