Information Technology Reference
In-Depth Information
Jim Scantlin, Director of Enterprise Information Management, states: “At Wal-Mart, we
never underestimate the importance of investing in innovative solutions that will improve
our ability to understand and anticipate our customers' needs.” 24
When determining the best method of data storage, the best overall solution is likely a
combination of different storage options. Secondary storage , also called permanent storage ,
serves this purpose.
Compared with memory, secondary storage offers the advantages of nonvolatility, greater
capacity, and greater economy. On a cost-per-megabyte basis, most forms of secondary stor-
age are considerably less expensive than primary memory (see Table 3.3). The selection of
secondary storage media and devices requires understanding their primary characteristics—
access method, capacity, and portability.
secondary storage (permanent
storage)
Devices that store larger amounts of
data, instructions, and information
more permanently than allowed
with main memory.
Table 3.3
Description
Cost
Storage Capacity (GB)
Cost Per GB
Cost Comparison for Various
Forms of Storage
All forms of secondary storage cost
considerably less per megabyte of
capacity than SDRAM, although they
have slower access times. A data
cartridge costs about $.21 per
gigabyte, while SDRAM can cost
around $49 per gigabyte—over
200 times more expensive.
(Source: Office Depot Web site,
www.officedepot.com, January 18,
2008.)
72 GB DAT 72 data cartridge
$14.95
72
$0.21
10 - 4.7 GB DVD+R disks
$9.95
47
$0.21
20 GB 4 MM backup data tape
$16.99
20
$0.85
120 GB portable hard drive
$139.99
120
$1.16
25 GB Rewritable Blu-ray disk
$29.99
25
$1.20
9.1 GB Write Once Read Many optical disk
$69.95
9.1
$7.69
1 GB flash drive
$7.99
1
$7.99
512 MB DDR2 SDRAM memory upgrade
$24.99
0.512
$48.81
As with other computer system components, the access methods, storage capacities,
and portability required of secondary storage media are determined by the information
system's objectives. An objective of a credit card company's information system, for example
MasterCard or Visa, might be to rapidly retrieve stored customer data to approve customer
purchases. In this case, a fast access method is critical. In other cases, such as equipping the
Coca-Cola field salesforce with pocket-sized personal computers, portability and storage ca-
pacity might be major considerations in selecting and using secondary storage media and
devices.
Storage media that allow faster access are generally more expensive than slower media.
The cost of additional storage capacity and portability vary widely, but they are also factors
to consider. In addition to cost and portability, organizations must address security issues to
allow only authorized people to access sensitive data and critical programs. Because the data
and programs kept in secondary storage devices are so critical to most organizations, all of
these issues merit careful consideration.
sequential access
A retrieval method in which data
must be accessed in the order in
which it is stored.
Access Methods
Data and information access can be either sequential or direct. Sequential access means that
data must be accessed in the order in which it is stored. For example, inventory data might
be stored sequentially by part number, such as 100, 101, 102, and so on. If you want to
retrieve information on part number 125, you must read and discard all the data relating to
parts 001 through 124.
Direct access means that data can be retrieved directly, without the need to pass by other
data in sequence. With direct access, it is possible to go directly to and access the needed
data—for example, part number 125—without having to read through parts 001 through
124. For this reason, direct access is usually faster than sequential access. The devices used
only to access secondary storage data sequentially are simply called sequential access storage
devices (SASDs) ; those used for direct access are called direct access storage devices
(DASDs) .
direct access
A retrieval method in which data can
be retrieved without the need to read
and discard other data.
sequential access storage
device (SASD)
A device used to sequentially access
secondary storage data.
direct access storage device
(DASD)
A device used for direct access of
secondary storage data.
 
 
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