Compaq 307560-001 Disk Subsystem Performance and Scalability - Page 13

WHITE PAPER (cont.) Note: The File System Cache data is stored in memory. Accesses to this data takes place over the Host Bus. ECG025.0997 ... I/O Bus Transfer Rates The I/O Bus consists of one or more of the following: Peripheral Component Interconnect (PCI), Dual Peer PCI, Extended Industry Standard Architecture (EISA) or Industry Standard Architecture (ISA). Table 6 defines these I/O bus types and their transfer rates. Table 6: I/O Bus Transfer Rates I/O Bus Type Definition and Transfer Rate Peripheral Component Interconnect (PCI) A system I/O bus architecture specification that supports 32-bit bus-mastered data. Designed to support plug-and-play configuration of optional peripherals. Transfers at a maximum rate of 133 MB/s. Dual Peer PCI (Supported on the ProLiant 5000, 6000, 6500 and 7000) A system I/O bus architecture specification that supports 32-bit bus-mastered data. Designed to support plug-and-play configuration of optional peripherals. Each controller transfers at a maximum rate of 133 MB/s, with a combined total throughput of 266 MB/s. Extended Industry Standard Architecture (EISA) A system I/O bus architecture specification that supports 8-, 16and 32-bit data throughput paths. Supports bus-mastering on 16and 32-bit buses. Transfers at a maximum rate of 33 MB/s. Industry Standard Architecture (ISA) A system I/O bus architecture specification that supports 8- and 16-bit data throughput paths. Supports bus-mastering on 16-bit buses. Transfers at a maximum rate of 8 MB/s. The theoretical threshold for the PCI Bus has a transfer rate of 133 MB/s. Because the PCI Bus can transfer data so quickly, it is the second least (file system cache being the first) likely of all of the disk subsystem components to be a performance bottleneck. To illustrate the point, it would take a minimum of three Compaq Wide-Ultra SCSI Controllers running at their maximum sustained transfer rate of 40 MB/s each to maintain throughput on the PCI Bus. Even running this configuration (3 x 40 MB/s = 120 MB/s) does not completely saturate the PCI Bus, having the capability of transferring at a rate of 133 MB/s. File System and Disk Controller Caching Transfer Rates File system and disk controller caching plays a fundamental role in system performance. Accessing data in memory, also known as Random Access Memory (RAM), is extremely fast (refer to Table 7 for transfer rates). Accessing data on the disk is a relatively slow process. If, in theory, we could avoid disk access by requesting and retrieving data from memory or "cache", system performance would improve dramatically. 13