Off-Line Backup Storage; Basic Virtual Memory Concepts; Virtual Memory In Simple Terms - Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

Chapter 4. Physical and Virtual Memory
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 keep in mind
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 varies 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 not currently loaded in the storage device. This situation is alleviated
somewhat by the use of robotic devices capable of automatically loading and unloading media, but
the media storage capacities of such devices are still finite. Even in the best of cases, access times are
measured in seconds, which is much longer than the relatively slow multi-millisecond access times
typical 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 a 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,
to run this application, there must be 15000 bytes of RAM available; even one byte less, and the
application would 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.
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