However, clusters are typically built by using shared disks linked to multiple “nodes” (computer systems). A high-speed interconnect between systems provides a means of exchanging data and instructions without writing to disk (see Figure 12-3). Each system runs its own copy of an operating system and Oracle instance. Grids, described later in this chapter, are typically made up of a few very large clusters. CPU with L1 Cache CPU with L1 Cache L2 Cache L2 Cache Memory Memory Network Connection I/O I/O Disk Cabling Disk Disk Figure 12-3. Typical cluster (two systems shown) Clusters | 295 Oracle’s support for clusters dates back to the VAXcluster. Oracle provided a sophisticated locking model so that the multiple nodes could access the shared data on the disks. Clusters require such a locking model because each machine in the cluster must be aware of the data locks held by other, physically separate machines in the cluster. Today, that Oracle solution has evolved into Real Application Clusters (RAC) (replacing the Oracle Parallel Server (OPS) that was available prior to Oracle9 i).
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