Storage Management

provides a uniform, logical view of information storage. The operating system
abstracts from the physical properties of its storage devices to define a logical
storage unit, the file. The operating system maps files onto physical media and
accesses these files via the storage devices.

File-System Management


File management is one of the most visible components of an operating system.
Computers can store information on several different types of physical media.
Magnetic disk, optical disk, and magnetic tape are the most common. Each
of these media has its own characteristics and physical organization. Each
medium is controlled by a device, such as a disk drive or tape drive, that capacity', data-transfer rate, and access method (sequential or random).
A file is a collection of related information defined by its creator. Commonly,
files represent programs (both source and object forms) and data. Data files may
be numeric, alphabetic, alphanumeric, or binary. Files may be free-form (for
example, text files), or they may be formatted rigidly (for example, fixed fields).
Clearly, the concept of a file is an extremely general one.
The operating system implements the abstract concept of a file by managing
mass storage media, such as tapes and disks, and the devices that control them.
Also, files are normally organized into directories to make them easier to use-
Finally, when multiple users have access to files, it may be desirable to control
by whom and in what ways (for example, read, write, append) files may be
accessed.
The operating system is responsible for the following activities in connection
with file management:

•  Creating and deleting files
•  Creating and deleting directories to organize files
•  Supporting primitives for manipulating files and directories
•  Mapping files onto secondary storage
•  Backing up files on stable (nonvolatile) storage media

Mass-Storage Management

As we have already seen, because main memory is too small to accommodate

all data and programs, and because the data that it holds are lost when power

is lost, the computer system must provide secondary storage to back up main

memory. Most modern computer systems use disks as the principal on-line

storage medium for both programs and data. Most programs—including

compilers, assemblers, word processors, editors, and formatters—are stored

on a disk until loaded into memory and then use the disk as both the source

and destination of their processing. Hence, the proper management of disk

storage is of central importance to a computer system. The operating system is

responsible for the following activities in connection with disk management:

• Free-space management
• Storage allocation
• Disk scheduling

Because secondary storage is used frequently, it must be used efficiently. The

entire speed of operation of a computer may hinge on the speeds of the disk

subsystem and of the algorithms that manipulate that subsystem.

There are, however, many uses for storage that is slower and lower in cost

(and sometimes of higher capacity) than secondary storage. Backups of disk

data, seldom-used data, and long-term archival storage are some examples. Magnetic tape drives and their tapes and CD and DVD drives and platters are

typical tertiary storage devices. The media (tapes and optical platters) vary

between WORM (write-once, read-many-times) and RW (read-write) formats.

Tertiary storage is not crucial to system performance, but it still must

be managed. Some operating systems take on this task, while others leave

tertiary-storage management to application programs. Some of the functions

that operating systems can provide include mounting and unmounting media

in devices, allocating and freeing the devices for exclusive use by processes,

and migrating data from secondary to tertiary storage.