UNIX System Administration Handbook - Evi Nemeth [78]
Computer vendors didn’t much care for this turn of events. In several highly publicized lawsuits, they attempted to protect interface specifications as trade secrets or as patented technology. Ultimately, the question became moot as the industry migrated toward standard interface technologies.
These days only a few interface standards are in common use, although several new technologies are on the horizon. It’s important to select disk drives that match the interfaces of the system on which they will be installed. If a system supports several different interfaces, you should use the one that best meets your requirements for speed, redundancy, mobility, and price.
• SCSI is one of the most common and widely supported disk interfaces. It comes in several flavors, all of which support multiple disks on a bus and various speeds and communication styles. SCSI is described in more detail in the next section.
• IDE was developed as a simple, low-cost interface for PCs. It was originally called “Integrated Drive Electronics” because it put the hardware controller in the same box as the disk platters and used a relatively high-level protocol for communication between the computer and the disks. This is now the standard architecture for all modern disks, but the name lives on. IDE disks are medium in speed, high in capacity, and unbelievably cheap. However, the interface design makes IDE a practical option only for workstations with four or fewer devices. See page 124 for more information about IDE.
• Fibre Channel is a serial interface that is gaining popularity in the enterprise environment due to its high bandwidth and to the large number of devices that can be attached to it at once. Fibre Channel devices connect together with a fiber optic or twinaxial copper cable. Current speeds are 100 MB/s and up. Common topologies include loops, called Fibre Channel Arbitrated Loop (FC-AL), and fabrics, which are constructed with Fibre Channel switches. Fibre Channel can speak several different protocols, including SCSI and even IP. Fibre Channel devices are identified by a hardwired ID number called a World Wide Name that’s similar to an Ethernet MAC address.
• The Universal Serial Bus (USB) has become popular for connecting devices such as keyboards and mice, but it has enough bandwidth to support slower disk devices such as removable hard disks and CD-ROM drives. USB is common on PCs and enables you to easily move a disk among systems.
SCSI and IDE are by far the dominant players in the disk drive arena. They are the only interfaces we will discuss in detail.
The SCSI interface
Several chip sets implement the SCSI standard, so vendors sometimes put SCSI support right on the CPU or peripheral board. SCSI defines a generic data pipe that can be used by all kinds of peripherals. Most commonly, it’s used for disks, tape drives, scanners, and printers. The SCSI standard does not specify how a disk is constructed or laid out, only the manner in which it communicates with other devices.
The SCSI standard has been through several revisions, with SCSI-3 being the current version. SCSI-1 was developed in 1986 as an ANSI standard based on the Shugart Associates System Interface (SASI), which was a commercially available system bus. SCSI-2 was developed in 1990. It is backward compatible with SCSI-1 but adds several performance features. These features include command queuing, which allows devices to reorder I/O requests to optimize throughput, and scatter-gather I/O, which permits Direct Memory Access (DMA) from discontiguous memory regions.
You might see the terms “fast” and “wide” applied to SCSI-2 devices, which means that the bus speed is doubled or that the