Unformatted text preview: disks.
2. Automated tape library, which uses a set of magnetic tapes.
3. CD-ROM jukebox, which uses a set of CD-ROMs.
These three types of mass storage devices are briefly described below.
The average access times of mass storage devices are measured in seconds instead of
milliseconds because a transport mechanism must first move to retrieve the storage media
(such as disk, tape or CD-ROM) upon which the desired data is stored. It requires several
seconds to locate the storage media specified and then few milliseconds are needed to
transfer the data to the memory. Thus an average access time of few seconds is common
for these storage devices. But a mass storage device has huge storage capacity and a very
small cost per bit stored.
Relatively slow access times limit the use of mass storage devices in many applications.
However, mass storage devices are cost-effective alternative to on-line magnetic tape or
disk storage in applications that require huge storage capacity and in which rapid access
to data is not essential. When used for off-line storage, mass storage devices are often
referred to as archival storage because of the very large volumes of historical or backup
data that they can store.
A disk array, which first appeared on the market in 1993, is a set of hard disks and hard
disk drives with a controller mounted in a single box. All the disks of a disk array form a
single large storage unit. A disk array is commonly known as a RAID (Redundant Array
of Inexpensive Disks). The term inexpensive comes from the fact that each of the
medium-sized hard disks in the disk array is much less expensive than a single large hard
Although RAID systems were originally developed to provide large secondary storage
capacity with enhanced performance, today they are also becoming popular due to
enhanced reliability. How do RAID systems provide enhanced storage capacity,
enhanced performance, and enhanced reliability is briefly explained below.
Suppose we have a 4 Gigabyte disk whose each track contains 128 sectors, each of 512
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This document was uploaded on 04/07/2014.
- Spring '14