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6.1 / MAGNETIC DISK 185

KEY POINTS

◆ Magnetic disks remain the most important component of external memory.
Both removable and fixed, or hard, disks are used in systems ranging from
personal computers to mainframes and supercomputers.
◆ To achieve greater performance and higher availability, servers and larger
systems use RAID disk technology. RAID is a family of techniques for
using multiple disks as a parallel array of data storage devices, with redun-
dancy built in to compensate for disk failure.
◆ Optical storage technology has become increasingly important in all types
of computer systems. While CD-ROM has been widely used for many
years, more recent technologies, such as writable CD and DVD, are becom-
ing increasingly important.

This chapter examines a range of external memory devices and systems.We begin with
the most important device, the magnetic disk. Magnetic disks are the foundation of ex-
ternal memory on virtually all computer systems.The next section examines the use of
disk arrays to achieve greater performance,looking specifically at the family of systems
known as RAID (Redundant Array of Independent Disks).An increasingly important
component of many computer systems is external optical memory,and this is examined
in the third section. Finally, magnetic tape is described.

6.1 MAGNETIC DISK

A disk is a circular platter constructed of nonmagnetic material, called the substrate,
coated with a magnetizable material. Traditionally, the substrate has been an alu-
minum or aluminum alloy material. More recently, glass substrates have been intro-
duced.The glass substrate has a number of benefits, including the following:
• Improvement in the uniformity of the magnetic film surface to increase disk
reliability
• A significant reduction in overall surface defects to help reduce read-write errors
• Ability to support lower fly heights (described subsequently)
• Better stiffness to reduce disk dynamics
• Greater ability to withstand shock and damage

Magnetic Read and Write Mechanisms
Data are recorded on and later retrieved from the disk via a conducting coil named
the head; in many systems, there are two heads, a read head and a write head. During
a read or write operation, the head is stationary while the platter rotates beneath it.
The write mechanism exploits the fact that electricity flowing through a coil
produces a magnetic field. Electric pulses are sent to the write head, and the resulting