There is more to it than that, however. It is important to note
as well that a computer does not think, that a computer has speed
and power out of proportion to anything else in our day-to-day
lives, and that a computer can be used to "enormously increase
the production and income of an area." I really like the
viewpoint expressed by this last quotation, as it tends to perk
up the ears and interest people in the real potential of this
marvelous machine. The quote is from an essay entitled What
is a Computer? by L. Ron Hubbard, which is included in full
in Appendix A.
The computers with which we are specifically concerned are those
which run on the OpenVMS operating system, namely the VAX and
the Alpha AXP.
The Alpha AXP computer was introduced by Digital in November 1992.
Although Alpha AXP processors use different sets of instructions
than VAX processors, Digital developed a translation of the OpenVMS
operating system for the new Alpha AXP computers. For reasons
which will become clear later on in this book, this had the effect
of transferring the very problem about which this book is written
from the VAX computer to the Alpha AXP computer; hence our discussion
applies equally to both computer systems.
Disks come in many different sizes and architectures.
And, even though we will limit our discussion to fixed (hard,
not floppy) disks, there are still a lot of sizes and architectures
to consider. Disk architecture is so fundamental to a discussion
of fragmentation that an entire chapter (the first) is devoted
to it. The chapter consists mostly of definitions of terms, but
it contains a lot of pictures and is worth reviewing even if you
are already familiar with basic disk concepts.
The purpose of a disk is to store information. This information,
or data, is said to be stored in "files."
In computer terminology, any collection of data that is treated
as a single unit on a storage medium (such as a disk) is referred
to as a "file." Not unlike a manila folder designed
to hold sheets of paper in concept, computer files are stored
on a disk, with the disk acting as a filing cabinet.
A file can be accessed (found and retrieved), modified (changed
in some way) and again stored on the disk. In this way, thousands
upon thousands of pieces of information can be stored on a physically
small disk, much more than can be stored in a regular filing cabinet.
Now we come to the real question: What exactly is fragmentation
anyway?
The word fragmentation means "the
state of being fragmented." The word fragment means
"a detached, isolated or incomplete part." It is derived
from the Latin fragmentum, which in turn is derived from
frangere, meaning "break." So fragmentation
means that something is broken into parts that are detached,
isolated or incomplete.
There are two types of fragmentation with which we are immediately concerned: file fragmentation and free space fragmentation. File fragmentation concerns computer disk files that are not whole but rather are broken into scattered parts, while free space fragmentation means that the empty space on a disk is broken into scattered parts rather than being collected all in one big empty space. File fragmentation causes problems with accessing data stored in computer disk files, while free space fragmentation causes problems creating new data files or extending (adding to) old ones.
In Figure 1, a file consisting of ten records is shown with all
ten records contiguous (immediately adjacent to each other) and
again with the records scattered around in three different groups.
The first arrangement is not fragmented. The second arrangement
is fragmented.
Taken together, we refer to the two types of fragmentation as
disk fragmentation. It is important to note that, when
talking about fragmentation, we are talking about the file as
a container for data and not about the contents (data) of the
file itself. People sometimes use the word fragmentation to
describe the condition of a file which has its records (contents)
scattered about within the file, separated by numerous small gaps.
This type of fragmentation may be a problem with the application
which maintains the file; it is not inherent in the operating
system or disk file structure.
Since record space fragmentation is the concern of applications
and not of the operating system or file system, this book does
not address the subject any further.
The various solutions to fragmentation are collectively referred
to as defragmentation, meaning something that gets rid
of the fragmentation problem.
This book will tell you all about fragmentation and defragmentation,
and do it in a way that is clear and understandable.
The material will be presented with care taken to define technical
terms. Lots of diagrams and pictures will be used and the material
will be presented on a gradient, bit by bit from the simplest
material to the more complex, so you do not have to be an expert
in later areas to understand earlier areas. For your part, take
care to look up the definitions of any words you do not understand
or are uncertain of, even non-technical words. If you are unfamiliar
with the physical objects discussed, arrange to look at or touch
them. For intangibles, of which there are many in this subject,
try drawing pictures of them to develop a more solid concept.
Finally, if you are having trouble in an area, go back to where
you were last doing well, and check to see if there were any words
you did not fully understand. If so, look up the definition in
a good (not complicated) dictionary. If you do this thoroughly,
you will find the material much easier to read. (This works for any subject.)
Here is the approach that will be used to present the story of
fragmentation:
After explaining how a disk works, and taking a quick look at
the OpenVMS file system, fragmentation will be explained in detail.
Then you will learn how to tell whether your system suffers from
fragmentation, and if so, how badly. We will then look at the
effects of fragmentation on the VAX or Alpha AXP system - what's
wrong with fragmentation? After that, we'll see what you can do
about it, and how you can get your computer to take care of fragmentation
by itself. This will include a view to the future, anticipating
the extinction of fragmentation for OpenVMS computer systems.
The final chapter is devoted to the ultimate solution to the fragmentation
problem. Along the way, we will view how this problem came about,
why it wasn't detected and prevented early on, Digital's viewpoint,
some of the controversy over defragmentation, and some of my personal
opinions on the whole subject.
Appendices are included with fill-in-the-blanks solutions for
organizational problems that get in the way of solving the fragmentation
problem: how to determine the cost of fragmentation, how to justify
the cost of a solution and how to get your company to agree to
spend the money you need to handle it.
The single most important thing to keep in mind is that a computer
is used to get work done, so fragmentation is not a problem
unless it interferes with doing useful work. Similarly, any solution
must be one which allows us to get more useful work done with
our computer system.
Actually, fragmentation does interfere with our use of
the computer to do work. It slows things down - little by little,
more and more. If left unhandled for a year or more, it can bring
any computer running the OpenVMS operating system close to a complete
standstill. I have personally experienced a VAX with a system disk so
badly fragmented it took half an hour just to log in to a user
account! After defragmenting, it took only a few seconds to log
in - good as new.