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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk External Performance Factors | PC System Factors ]

Interface Bus Speed

Every hard disk read or write involves a sequence of data transfers. Looking at a read: first the data is retrieved from the hard disk platters. It is transferred to the drive's internal cache by its controller. Then the data is sent over the interface cable to the interface controller on the PC system. That controller resides on a system bus, and uses the system bus to communicate the data to the CPU and the rest of the PC. System buses are discussed in detail in their own section.

Normally the speed of the bus used for the hard disk interface is not something that you really need to be concerned with. Virtually all systems today use the PCI bus for interfacing to their hard disks and other storage devices, which is fast enough to handle even the high interface transfer rates of modern drives. (Even if your IDE cables plug into the motherboard directly, they are still going to an IDE controller chip that logically "resides" on the PCI bus.) However, as the interface transfer rate of IDE/ATA drives (maximum 100 MB/s) now approaches the limits of the PCI bus (about 127 MB/s), at some point this will become an issue even on new systems; probably within two or three years. Hard disks continue to get faster and faster.

On older systems interface bus speed limits can become a real issue. There are still systems around that use ISA bus hard disk controllers, for example. Even if you could get one of these older cards to work with a large, modern drive, the slow speed of the ISA bus would drag it down. ISA is limited to a maximum bus bandwidth of under 16 MB/s, easily exceeded even for sustained transfers by most any modern drive, not to mention burst transfers!

Even on new systems, alternative means of interfacing hard disks can have a major impact on performance. The widespread adoption of the universal serial bus (USB) standard has been a boon for portability of devices and easy interchanging of hardware. Some companies are now even offering USB-based hard disks. These are convenient, but the slow speed of the USB interface--which was designed for slow items like scanners and keyboards, not hard disks--effectively cripples these drives, limiting them to a maximum transfer rate of about 1 MB/s. That may be OK for moving data between PCs, archiving seldom-needed data or doing smallish backups, but it's very slow for just about anything else!

Next: System BIOS Issues

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