Page 245 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
Objectives

After studying this unit, you will be able to:
• Explain meaning of secondary storage structure

• Understand the benefits of secondary storage
• Discuss disk structure
• Explain disk scheduling

• Understand disk management
• Explain swap-space management
• Understand overview of RAID structure

• Discuss improvement of reliability via redundancy
• Explain improvement in performance via parallelism

Introduction

Any non-volatile storage medium that is not directly accessible to the processor. Memory directly
accessible to the processor includes main memory, cache and the CPU registers. Secondary
storage includes hard drives, magnetic tape, CD-ROM, DVD drives, floppy disks, punch cards
and paper tape.
Secondary storage devices are usually accessed via some kind of controller. This contains registers
that can be directly accessed by the CPU like main memory (“memory mapped”). Reading and
writing these registers can cause the device to perform actions like reading a block of data off
a disk or rewinding a tape.

7.1 The Benefits of Secondary Storage

Picture, if you can, how many filing-cabinet drawers would be required to hold the millions
of files of, say, tax records kept by the Internal Revenue Service or historical employee records
kept by General Motors. The record storage rooms would have to be enormous. Computers,
in contrast, permit storage on tape or disk in extremely compressed form. Storage capacity is
unquestionably one of the most valuable assets of the computer.
Secondary storage, sometimes called auxiliary storage, is storage separate from the computer
itself, where you can store software and data on a semi permanent basis. Secondary storage
is necessary because memory, or primary storage, can be used only temporarily. If you are
sharing your computer, you must yield memory to someone else after your program runs; if
you are not sharing your computer, your programs and data will disappear from memory when
you turn off the computer. However, you probably want to store the data you have used or
the information you have derived from processing; that is why secondary storage is needed.
Furthermore, memory is limited in size, whereas secondary storage media can store as much
data as necessary. Keep in mind the characteristics of the memory hierarchy that were described
in the section on the CPU and memory.

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