Beautiful Architecture

Input/Output

Each processor has a single I/O bus with up to 32 controllers. All controllers are dual-ported and connected to two different CPUs. At any one time, only one CPU has access to any specific controller. This relationship between CPU and controller is called ownership: the controlling CPU “owns” the controller. The backup path is not used until the primary path fails or the system operator manually switches to it (a so-called primary switch).

Disks are a particularly sensitive issue because many components could fail. It could be a disk itself, the physical connection (cable) to the disk, the disk controller, the I/O bus, or the CPU to which it is connected. As a result, in addition to the dual-ported controllers, each disk is physically duplicated—at least in theory—and it is also dual-ported and connected to two different controllers, both connected to the same two CPUs The restriction remains that only one CPU can access each controller at any one time, but it is possible for one of the CPUs to own one of the controllers and the other CPU to own the other controller. This is also desirable from a performance point of view.

Figure 8-1 shows a typical configuration: as the gray highlighted paths indicate, the I/O process for the system disk $SYSTEM accesses it via CPU 0 and the first disk controller, while the I/O process for another disk $DATA, connected to the same two controllers, accesses the disk via CPU 1 and the second disk controller. CPU 0 “owns” the first controller, ...