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Enhancing PC Disk Performance
Bruce Schafer
Multisoft Corporation
May 22, 1986
Introduction
From the beginning, the main benefit of personal computers
has been increased productivity. Just as we become familiar
with the advantages of personal computers we begin to notice
that we spend an increasing amount of time waiting for the
computer to finish its task. This is partly due to the
natural human tendency to always want more. But, there is a
more objective reason as well. Usually, when we begin an
activity on a computer, we begin small -- a small document, a
small spreadsheet, a small database, or a few weeks of
accounting information. Inevitably the data grows, resulting
in slower responses from the computer, especially when disk
access is involved.
This article discusses a way of improving the speed of
personal computers using a software technique called disk
caching. Put simply, a disk cache is a collection of copies
of recently used disk sectors. By keeping a copy of recently
used disk sectors in extra RAM memory, a disk cache can
reduce the number of times the application actually accesses
the disk. This, in turn, can dramatically speed up the
performance of the application.
History
In the early days of computing, memory was a very expensive
resource. Large rooms were filled with many hundreds of
thousands of dollars of memory, and still there never seemed
to be enough. Much research money went into developing the
concept of virtual memory. (See "Virtual Memory" by Peter J.
Denning in Computing Surveys, September 1970). Much of this
research amounted to answering the question, "How can I get
my programs to think I have more memory than I actually do
without losing to much in performance?" Much of this
research into minimizing the loss of performance in creating
virtual memory applies equally well to increasing the
performance of disk transfers.
Since the time large virtual memory systems were developed,
the price of memory has dropped dramatically. Just in the
short time since the IBM PC was introduced, we have seen
memory boards go from 64K bytes per board to 256K/384K, and
more recently to 2048K. The price of these boards at the
time they were introduced was about the same in each case.
In short, the price of personal computer memory has dropped
by at least 10-to-1 in four years. This dramatic drop in the
cost of memory provides an opportunity to use memory in ways
that would have previously been too expensive. In
particular, RAM memory can be used to create a disk cache
that dramatically improves the performance of applications
which access disk storage.
Related Technologies
You might ask why isn't disk caching built into MSDOS? In
fact, we might anticipate that it will be added in some
future version of DOS. At the time DOS was originally
designed, however, it was clearly out of the question. Memory
was still too expensive to seriously consider it. On the
other hand, DOS did provide for a related technique, namely
buffering. Starting with Version 2.0, DOS even gave the user
the option of specifying the number of buffers to be used for
disk access. (If you are not familiar with this command,
look up the BUFFERS command in your DOS manual.) While
buffering provides some performance improvement, it falls far
short of what can be accomplished by a disk cache.
Another technology related to disk caching is called RAM
disk. The idea of RAM disk is similar in that it takes
advantage of extra memory to increase disk performance. The
major difference is that a RAM disk creates a completely
separate virtual disk. A typical use of a RAM disk involves
copying frequently used programs to the RAM disk at the
beginning of a session. Later, when it is time to run one of
these programs, they are loaded from the RAM disk rather than
the conventional disk. As a result, they begin executing in
a fraction of the normal time. As you can see, this approach
is particularly beneficial when the same programs are loaded
several times in a session. The case for using a RAM disk for
data is not nearly so clear cut, however. First of all, not
all of your data may fit in the RAM disk. Secondly, if you
make any changes to the data during a session, you must
remember to copy it to a conventional disk when you are done.
Thirdly, you suffer the risk that the power may fail before
you get around to copying your data back to a real disk.
Because of these disadvantages, very few people use RAM disks
for data.
In contrast, a disk cache suffers from none of these
disadvantages. By its nature, a disk cache only requires
enough memory to hold the most frequently used parts of the
data. Changes to the data are automatically written to the
physical disk, eliminating the inconvenience and risk
inherent in postponing this step when using a RAM disk. Even
in the case of program loading, a disk cache is often the
superior solution. This is because it eliminates the
requirement of anticipating which programs will be used in a
session and copying them to a RAM disk. In summary, a disk
cache provides almost all of the advantages of a RAM disk and
it does so automatically.
Disk Cache Theory
As mentioned earlier, a disk cache works by accumulating
copies of frequently used sectors in memory. As new sectors
are accessed by applications it automatically keeps a copy of
them as well. Obviously, there won't always be room for a
copy of a new sector as well as all previously used sectors.
In this case a replacement algorithm is needed. A
replacement algorithm is the computer science term for the
answer to question, "Which sector copy gets thrown away?"
(Keep in mind that any changes to the sector have already
been written out to the physical disk.) Early research into
virtual memory and other types of hierarchical storage showed
that a vast majority of the time, a Least Recently Used
approach was best. In other words, replace the copy of the
least recently used sector with a copy of the most recently
used sector.
Disk Cache Practice
So how does disk caching affect actual applications? The
answer, of course, varies with the application and what kind
of disk you are using in your system. Disk caches provide
the most dramatic improvement in diskette based systems.
Retrieval times in a database can be improved by a factor of
six or more and sorts can be improved by a factor of two.
Even in hard disk systems the improvement is significant --
retrievals can be improved by a factor of two and sorts by
almost a factor of two. You may be wondering about the
impact on faster hard disks typical of AT configurations.
Even with these disks, the performance boost of disk cache
can still be as much as a factor of three-to-one. This is
partly due to the fact that the disk cache function is being
provided by the faster 80286 processor in the AT.
Are performance improvements restricted to data bases?
Certainly not -- a broad range of applications benefit: DOS
batch files run faster; document and spreadsheet retrievals
improve; and compilers and accounting packages speed up. The
exact improvement varies according the nature of the
application's use of the disk, the amount of data being used,
and how often that data is accessed.
Another question that comes up with a disk cache is how big
should they be? The easy answer is the bigger the better. A
slightly better answer is to give the disk cache whatever
memory your applications don't require. You should also find
that you will need fewer DOS buffers when using a disk cache.
The memory taken up by DOS buffers is probably better
allocated to the cache.
To be more specific concerning disk cache size, 64K will
usually provide a noticeable improvement and 128K an
impressive improvement. If you are using a large database or
a large word-processing file, an even bigger disk cache may
be desirable. This is also true if you frequently switch
from one application to another.
Products on the Market
Over the last year, several disk cache products have appeared
on the market. They all provide the same basic function of
providing a variable size disk cache. While each of them has
several optional features, it is beyond the scope of this
article to compare and contrast them. Following is a list of
disk cache products and prices readily available at the time
this article was written. Except as noted the products are
not copy protected.
1. Flash. $49.95. Software Masters, 6223 Carrollton Ave.,
Indianapolis, IN 46220 (317) 253-8088.
2. Personal PC-Kwik. $39.95. Multisoft Corporation, 18220
S.W. Monte Verdi Blvd., Beaverton, OR 97007, (503)
642-7108.
3. Lightning. $49.95 with copy protection, $89.95 without.
Personal Computer Support Group, 11035 Harry Hines Blvd.,
#206, Dallas, TX 75229, (214) 351-0564.
4. Shareware PC-Kwik. Registration charge, $19.95.
Distributed by Multisoft Corporation through
personal-computer clubs and bulletin boards.
5. VCache. $65.00. Golden Bow Systems, 2870 Fifth Avenue,
Suite 201 San Diego, CA 92013., (619) 298-9349
Conclusion
The obvious way to increase the performance of your personal
computer is to purchase additional hardware options. You may
also want to consider upgrading to a more expensive model
sometime in the future. In the meantime, you should consider
using a disk cache. In so doing, you can save yourself both
time and money.
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