Chapter 4. Physical and Virtual Memory

47

Note

Although there is much more to learn about hard drives, disk storage technologies are discussed in

more depth in Chapter 5 Managing Storage. For the time being, it is only necessary to realize the

huge speed difference between RAM and disk based technologies and that their storage capacity

usually exceeds that of RAM by a factor of at least 10, and often by 100 or more.

4.2.5. Off Line Backup Storage

Off line backup storage takes a step beyond hard drive storage in terms of capacity (higher) and

speed (slower). Here, capacities are effectively limited only by your ability to procure and store the

removable media.

The actual technologies used in these devices can vary widely. Here are the more popular types:

Magnetic tape

Optical disk

Of course, having removable media means that access times become even longer, particularly when

the desired data is on media that is not currently in the storage device. This situation is alleviated

somewhat by the use of robotic devices to automatically load and unload media, but the media storage

capacities of such devices are finite. Even in the best of cases, access times are measured in seconds,

which is a far cry even from the slow multi millisecond access times for a high performance hard

drive.

Now that we have briefly studied the various storage technologies in use today, let us explore basic

virtual memory concepts.

4.3. Basic Virtual Memory Concepts

While the technology behind the construction of the various modern day storage technologies is truly

impressive, the average system administrator does not need to be aware of the details. In fact, there is

really only one fact that system administrators should always keep in mind:

There is never enough RAM.

While this truism might at first seem humorous, many operating system designers have spent a great

deal of time trying to reduce the impact of this very real shortage. They have done so by implementing

virtual memory   a way of combining RAM with slower storage to give the system the appearance

of having more RAM than is actually installed.

4.3.1. Virtual Memory in Simple Terms

Let us start with a hypothetical application. The machine code making up this application is 10000

bytes in size. It also requires another 5000 bytes for data storage and I/O buffers. This means that, in

order to run this application, there must be 15000 bytes of RAM available; even one byte less, and the

application will not be able to run.

This 15000 byte requirement is known as the application's address space. It is the number of unique

addresses needed to hold both the application and its data. In the first computers, the amount of

available RAM had to be greater than the address space of the largest application to be run; otherwise,

the application would fail with an "out of memory" error.