Managing RAID on Linux - Derek Vadala [26]
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 to the SCSI specification. Fast SCSI defined a 10 MHz signaling rate (yielding a 10 MB/s transfer rate) and from there, Ultra SCSI (20 MHz), Ultra2 SCSI (40 MHz), and Ultra3 SCSI (80 MHz) were eventually defined and implemented.
Although SCSI is governed by the American National Standards Institute (ANSI), some manufacturers, throughout SCSI's evolution, did not want to wait for newer and faster SCSI protocols to be standardized. In an attempt to gain market share, many SCSI manufacturers have prematurely released their own prestandardized implementations. The result, as with ATA, was a deviation in naming among manufacturers, although incompatibility was rare and today is generally a nonissue. Table 2-5 shows the various implementations of SCSI and their maximum data throughput rates.
Table 2-5. Overview of SCSI data throughput
Names
Bus width
Signaling rate (MHz)
Maximum data throughput (MB/s)
SCSI-1, SCSI, Narrow SCSI
8
5
5
Fast SCSI, Fast-Narrow SCSI
8
10
10
Fast Wide SCSI
16
10
20
Ultra SCSI
8
20
20
Ultra Wide SCSI
16
20
40
Ultra2 SCSI, Ultra2 Narrow SCSI
8
40
40
Ultra2 Wide SCSI
16
40
80
Ultra3 SCSI, Ultra 160 SCSI
16
80
160
There is already talk of yet higher signaling rates for SCSI. A wide bus with a signaling rate of 160 MHz, yielding a throughput of 320 MB/s, is currently under development. It is likely to be commonplace within the next year.
Transmission types
The final difference between SCSI implementations is found in the type of cabling used to interconnect devices. Single-ended (SE) devices transmit information over single wires. Using single wires for transmission on the disk bus limits the maximum cable length of the disk bus. It also limits the maximum data throughput because error correction requires a pair of wires for each signal.
Differential SCSI transmits information over a pair of wires, which requires more expensive cables, but solves the performance and cable length limitations imposed by single-ended SCSI. The first standard, high-voltage differential (HVD), provided a faster disk bus and used an extremely high voltage. HVD also allowed a maximum cable length of 25 meters, compared with the 6-meter maximum of SE devices. However, manufacturing controllers and devices that supported HVD dramatically increased hardware costs. The drastic increase in voltage means that a separate chip was required to regulate the voltage of the SCSI bus. It also made HVD and SE incompatible, requiring older devices to be replaced or connected to a separate