Characteristics of SCSI Disks

SCSI has always been the choice for high-speed hard disk operations. Characteristics of SCSI hard disks are the complement of EIDE characteristics:

• High cost SCSI drives are typically more expensive than EIDE drives of the same capacity. Some of this is due to the SCSI interface, some is due to economic factors, and some of it relates to drive performance.

• High performance SCSI hard disks have larger caches, faster rotation rates, and lower seek times, on average than EIDE drives of the same capacity.

• High command latency The SCSI protocols impose greater overhead on commands than do the EIDE protocols. SCSI drives are therefore a bit slower to respond to commands than are EIDE drives.

• High capacity You can obtain EIDE drives as large as most SCSI drives sold, but a few SCSI models exceed the capacity of the highest-capacity EIDE models.

• LBA mode only SCSI disks have always communicated with their host adapters by presenting a view of the disk using a single linear block of addresses. This greatly simplifies matters for the computer if the computer understands this view of things. Unfortunately, x86 OSs have traditionally used CHS addressing, so SCSI host adapters translate LBA to CHS mode.

At least as important as the differences between EIDE and SCSI hard disks are the differences between the EIDE and SCSI busses. Characteristics of the SCSI bus that set it apart from the EIDE bus include

• A 7- or 15-device limit In theory, you can attach up to 7 or 15 devices to each SCSI host adapter, depending upon the SCSI variant in use. (Wide SCSI variants support up to 15 devices, whereas narrow varieties support only 7.) In practice, cable length limits make it difficult to add more than 5 or 6 devices to a narrow chain. Each SCSI device has a unique ID number, which is set by a jumper or, in recent SCSI variants, can be assigned by the host adapter.

• Device re-use Because the limit on SCSI devices is much higher than the limit on EIDE devices, it's more practical to add hard disks to a SCSI system than it is to add hard disks to an EIDE system. This means your initial investment in hard drives can last longer.

Chapter 5

• Concurrent communication The SCSI bus was designed from the beginning to support concurrent communication. You can begin a transfer with one SCSI device and then start a second transfer with another device on the same chain. Both transfers can proceed at full speed, provided the sum of the speeds of the individual devices don't equal more than the speed of the SCSI bus. This factor alone gives SCSI a huge advantage when you need a high-performance disk system.

• Interrupt consumption Each SCSI host adapter, like each EIDE controller, requires one IRQ. Because you can attach many more SCSI devices to a SCSI chain, however, the SCSI devices tend to consume fewer IRQs.

• Termination A SCSI bus consists of several SCSI devices, including a SCSI host adapter, strung in a line. The device on each end of the line must be appropriately terminated, which is accomplished by setting a jumper or by adding a special terminating resistor pack to the SCSI cable. Termination problems account for many of the difficulties that arise with SCSI devices.

• Driver issues Unlike EIDE, SCSI controllers don't operate as clones of the early PC hard disk interfaces. Therefore, Linux needs specialized drivers for any SCSI host adapter you choose; there is no fallback compatibility mode as there is with EIDE. (Chapter 9, "SCSI Host Adapters," covers this matter in more detail.)

If costs were no object, SCSI would be the bus of choice for hard disks. Unfortunately, using SCSI rather than EIDE typically adds over $100 to the cost of a PC, and often $300-$500, depending upon the size of hard disk, number of SCSI devices, and so on. You do get more performance for this money, but the expense might not be justified for low-end or mid-range systems. I do strongly favor SCSI on systems that require several disk-like devices, such as tape backup units, CD-R drives, zip drives, and so on. These three devices alone, in conjunction with a CD-ROM drive and a single hard disk, require three IRQs and an add-on EIDE controller if bought in EIDE form. In SCSI form, all five devices can be attached to a single host adapter and consume a single IRQ. The latest SCSI host adapters, however, work best with the latest disk devices when only other high-speed SCSI devices are in use. It's sometimes therefore helpful to have two SCSI host adapters in such systems.

Continue reading here: Using EIDE and SCSI in a Single System

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