24 Part I • Information Technology
near the end. Thus, we would rarely try to find a single
record with a sequential access file. Instead, we would
accumulate a batch of transactions and process the entire
batch at the same time. (See the discussion of batch pro-
cessing in Chapter 5.)
Sequential access files are usually stored on magnetic
tape. A magnetic tape unitor magnetic tape drive is the
file device that stores (writes) data on tape and that
retrieves (reads) data from tape back into memory. Even
with batch processing, retrieval from magnetic tape tends
to be much slower than retrieval from direct access files.
Thus, if speed is of the essence, sequential access files
might not be suitable. On the other hand, magnetic tapes
can store vast quantities of data economically. For exam-
ple, a tape cartridge that can store up to 500 billion bytes
(gigabytes) of data can be purchased for under $160.
DIRECT ACCESS FILES Adirect access file,stored on a
direct access storage device (DASD),is a file from which it
is possible for the computer to obtain a record immediately,
without regard to where the record is located in the file.
There are several types of direct access devices, including
hard drives, removable disk drives, CD and DVD drives, and
flash drives. A typical hard drive for a computer consists of
a continuously rotating disk, where the disk resembles an
old-style phonograph record. An access mechanism (arm)
moves in and out on the disk to record on and read from
hundreds of concentric tracks on the disk surface. Typical
internal hard drives for microcomputers store from 160 bil-
lion bytes (gigabytes) to 2 trillion bytes (terabytes) and cost
from $50 to $200. The speed at which data can be read from
or written on a hard drive is quite fast, with transfer rates up
to 375 million bytes (megabytes) per second.
Gaining popularity in recent years are portable and
desktop external hard drives that can be attached to a
microcomputer’s USB port. Desktop external hard drives
have capacities and costs similar to internal hard drives, as
well as similar data transfer rates. These external drives can
provide a backup capability as well as a significant addition
to internal hard drive capacity. Compared to internal and
desktop external hard drives, portable drives typically have
a smaller maximum capacity (e.g., 640 gigabytes at a cost
of $150) and a slower data transfer rate (i.e., up to
60 megabytes per second). These portable drives, which
have a “drop guard” feature to prevent damage from drops,
permit the user to back up very large data files and move
these large data files from one computer system to another.
EMC Corporation, based in Hopkinton,
Massachusetts, has become the market leader in storage
systems for large computers by devising a way to link
together a large number of inexpensive, small hard drives
(such as those used in microcomputers) as a substitute for
the giant disk drives (containing a stack of disks) that
were previously used. As an example, EMC’s Symmetrix
DMX-4 950 model can be configured with from 32 to 360
hard drives, each with a storage capacity from 73 gigabytes
up to 1,000 gigabytes (1 terabyte), giving a total storage
capacity from 2.3 terabytes (trillion bytes) up to a maxi-
mum of 360 terabytes (EMC Web site, 2010).
In contrast to these fixed-disk file devices, direct
access devices can also employ a removable disk. For
instance, a removable 3.5-inch high-density disk for a
microcomputer can store up to 1.44 million bytes
(megabytes) of data and costs less than 50 cents. The disk
drive itself costs about $20. These 3.5-inch disks were very
popular in the 1980s and 1990s, but they have been largely
displaced in the twenty-first century by optical disks and
flash drives, to be discussed next.
Another type of removable disk is the optical disk,
which is made of plastic and coated with a thin reflective
alloy material. Data are recorded on the disk by using a
laser beam to burn microscopic pits in the reflective sur-
face, employing a binary coding scheme. There are two
primary types of optical disks in common use today: a
compact disk (CD)and a digital video disk,ordigital
versatile disk (DVD).Some CDs and DVDs are read-only
disks, some are recordable disks (can be written on once
and read many times), and some are rewritable (can be
written on many times and read many times). A CD has
much less capacity than a DVD: Standard capacity for a
CD is 700 megabytes of data, while capacity for a DVD is
8.5 gigabytes for a dual-layer DVD. As an example of a
DVD reader/writer for a microcomputer, Iomega’s Super
DVD Writer (which can also read and write CDs) is rated
22x (which translates to writing a DVD at 30 megabytes
per second) and costs about $90 (Iomega Web site, 2010).
The newest and smallest portable DASD for PCs
utilizesflash memory—as used in digital cameras and
portable music players—rather than a magnetizable disk.
Thisflash drivegoes by various names, including jump
drive, mini USB drive, or keychain drive. Keychain driveis
perhaps the most descriptive, because the device is not much
larger than the average car key—usually about 2^1 / 2 inches
long (see Figure 2.3). The maximum capacities of these
flash drives are increasing over time, and the prices are
going down; as an example, SanDisk’s 32-gigabyte Cruzer
USB flash drive sold for about $134 in 2010. These flash
drives, which are designed to plug into the standard USB
port on a PC, have a maximum sustained data transfer rate of
40 megabytes per second. The flash drive is an economical
and extremely convenient way to save and transport signifi-
cant amounts of data.
The key to the operation of direct access files is that
the physical file is divided into cells, each of which has an
address. The cells are similar to memory cells, except that
they are much larger, usually large enough to store several