Managing RAID on Linux

SCSI

The Small Computer Systems Interface, or SCSI, has been around much longer than ATA, but has traditionally been priced out of consumer reach. This changed in 1986 with the introduction of the Apple Macintosh II, which came standard with an SCSI controller, but no hard disk. The following year, Apple introduced the Mac SE and the Mac II, both available with optional internal hard disks.^[3]

Bus-width and signaling rates

SCSI, like the data bus of a motherboard, is defined by both a bus-width and a signaling rate (sometimes called the clock rate). Increasing either of these parameters increases the overall throughput of the SCSI bus. Bus-width is either narrow (8-bit) or wide (16-bit). As with motherboards, the bus-width determines how many bytes of data can be transmitted during each clock cycle. Bus-width also determines the number of devices that can be connected to a single SCSI bus. Narrow buses can handle eight devices and wide buses can handle sixteen. This gives each bus type 7 and 15 usable devices respectively (one device number is reserved for the controller).

The signaling rate measures how many times a second data can be pushed through the SCSI bus. Signaling rates are measured in megahertz. The first implementation of SCSI, also called SCSI-1, had a bus-width of 8 bits and a signaling rate of 5 MHz. One byte of data, transmitted five million times per second across the SCSI bus, gave SCSI-1 a data throughput of 5 MB/s. Since SCSI-1, more signaling rates have been added ...