Hard Drive Performance Characteristics; Mechanical/Electrical Limitations - Red Hat ENTERPRISE LINUX 3 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

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SCSI ID as the bus's controller. This also means that, in practice, only 7 (or 15, for wide SCSI) devices
may be present on a single bus, as each bus must reserve an ID for the controller.
Tip
Most SCSI implementations include some means of scanning the SCSI bus; this is often used to
confirm that all the devices are properly configured. If a bus scan returns the same device for every
single SCSI ID, that device has been incorrectly set to the same SCSI ID as the SCSI controller. To
resolve the problem, reconfigure the device to use a different (and unique) SCSI ID.
Because of SCSI's bus-oriented architecture, it is necessary to properly terminate both ends of the bus.
Termination is accomplished by placing a load of the correct electrical impedance on each conductor
comprising the SCSI bus. Termination is an electrical requirement; without it, the various signals
present on the bus would be reflected off the ends of the bus, garbling all communication.
Many (but not all) SCSI devices come with internal terminators that can be enabled or disabled using
jumpers or switches. External terminators are also available.
One last thing to keep in mind about SCSI — it is not just an interface standard for mass storage
devices. Many other devices (such as scanners, printers, and communications devices) use SCSI.
Although these are much less common than SCSI mass storage devices, they do exist. However, it is
likely that, with the advent of USB and IEEE-1394 (often called Firewire), these interfaces will be
used more for these types of devices in the future.
Tip
The USB and IEEE-1394 interfaces are also starting to make inroads in the mass storage arena;
however, no native USB or IEEE-1394 mass-storage devices currently exist. Instead, the present-
day offerings are based on ATA or SCSI devices with external conversion circuitry.
No matter what interface a mass storage device uses, the inner workings of the device has a bearing
on its performance. The following section explores this important subject.

5.4. Hard Drive Performance Characteristics

Hard drive performance characteristics have already been introduced in Section 4.2.4 Hard Drives;
this section discusses the matter in more depth. This is important for system administrators to under-
stand, because without at least basic knowledge of how hard drives operate, it is possible to unwittingly
making changes to your system configuration that could negatively impact its performance.
The time it takes for a hard drive to respond to and complete an I/O request is dependent on two
things:
The hard drive's mechanical and electrical limitations
•
The I/O load imposed by the system
•
The following sections explore these aspects of hard drive performance in more depth.
Chapter 5. Managing Storage