One of the things that seems to be promoted the most when
you look at new computers are the "blazing fast"
CD-ROM drives. The chances are that if you have one of the
scorchers, you have been disappointed with its performance.
I originally had a 12x CD-ROM and bought a 24x CD-ROM to replace
it and it is no where near 2x's as fast as the old one. What
is the deal with that?
There are two factors involved in a CD-ROM drive's performance.
The first is the DATA TRANSFER
RATE of the drive. This parameter is widely touted
by manufacturers and has been given a good deal of press.
We've gone from single speed drives with a data transfer rate
of 150 Kb/s to 32x drives at 4800 Kb/s (the 32x means that
it is 32 times faster than 150 Kb/s e.g. 150x32=4800). Speeding
up the data transfer rate of a drive is a relatively easy
thing to do. A drive's manufacturer designs a drive that simply
spins the CD-ROM faster. That's why there has been such a
big increase in the data transfer rate of drives in the past
few years. It's just not that big a feat to design a drive
that spins faster.
The other factor affecting a CD-ROM drives performance is
ACCESS TIME, also called
seek time. This is the average time it takes to reposition
a drive's read head to a new location and start reading data.
Access time has improved relatively little in the course of
developing new drives with faster and faster data transfer
rats. It's not that the drive manufacturers wouldn't like
to greatly increase access time, it's just not an easy thing
to do. The hard fact of the matter is you're up against the
laws of physics. Because drive access time hasn't improved
all that much, its role in drive performance has often been
downplayed. You'll hear someone brag about their new 32x drive,
but these same people probably don't know the access time
of this "speed demon". You can be sure, however,
that it is slow. It's interesting to contrast this situation
with that of hard drives. With hard drives, the performance
criteria people talk about is access time! It's the data transfer
rate they have no idea about. Even with 32x CD-ROM drives,
the average time it takes to reposition a drive's read head
to a new location and start reading data is at least 100 milliseconds.
Compare that with the under 10 millisecond access times of
today's hard drives. Access time is the other piece of the
CD-ROM drive performance puzzle.
Data on a CD-ROM is recorded in equal length sectors on a
single spiral track that starts at the inner edge of the disc
and finishes at the outer edge. This track is on the order
of .6 microns wide, with a turn spacing of 1.6 microns. A
micron, if you don't happen to remember, is equal to a millionth
of a meter. A human hair is about 100 microns in diameter.
A CD-ROM has a track density of about 16,000 turns per inch
if you measured across the disc. If you stretched this track
out into a straight line it would measure over 3 miles in
A drive's read head assembly consists of a solid state laser,
lenses, a beam splitting mirror and a photo diode to detect
the reflected light. In relative terms this assembly is massive.
The magnetic heads used in hard disk drives are tiny by comparison.
CD-ROM drives lower than about 12x, utilize a principle called
constant linear velocity (CLV). With CLV the data must pass
across the read head at the same speed, no matter where on
the disc it is positioned. Because data sectors on the outer
and inner edges are the same length, the rotational speed
of the disc must constantly change as the read head changes
position. If it didn't, the bits on the outer edge of the
disk would pass by almost 3 times faster than those near the
CD-ROM drives of 12x or higher, give up CLV operation to
improve access times. But this means that for a 12x drive,
12x is really the top speed. This rated speed is only attainable
on the outermost tracks of your CD-ROM. Depending on where
the read head is positioned, the data transfer rate will be
lower than 12x. In fact, since data starts at the inner edge
of a CD-ROM, it almost always will have a data transfer rate
slower than its rated speed! Your actual data transfer rate
may be as much as half the speed that it's rated for. This
means your 20x drive may only really transfer data at 10x!
So to summarize the mechanical problem we face, we must precisely
follow a very narrow track of data using a relatively massive
laser assembly. At the same time (for drives rated less than
12x), we must constantly vary the rotational speed of the
disc so the data passes at a constant linear speed when we
move the read head. And we want to be able to reposition this
read head and be ready to access new data as quickly as possible.
As you can imagine, it's just not possible to do this very
quickly. The more mass something has, the slower you can whip
it around. If in addition, you have to position it with sub-micron
accuracy, this takes more time. And if you also have to readjust
to a new rotational speed every time you move, you need more
This is why CD-ROM access times are slow and it's not going
to change any time soon. Although 24x and 32x speed drives
have increased the raw data rate (by increasing the rotational
speed), access times have not dropped proportionally.
How do you make them faster?
After reading about a program in PC
World Magazine called CD-Quick Cache, I contacted the
creator and he sent me a copy of the program to test. It works
great. I have no affiliation with Peter
Volpa, but I would still like to highly recommend this
program. It provides a lot of the answers.
The key to speeding up CD-ROM performance is to minimize
the number of times we reposition the read head. Because
every time we move this head, we waste time. The way to reduce
head movement is to use a cache like CD-Quick Cache. Once
data is read into the cache, it will be accessed from RAM
(or disk) instead of the CD. If we can anticipate data to
read when we are positioned at this data, we can minimize
head movement. CD-Quick Cache uses advanced caching techniques
to help ensure that the data you need most will be found in
Microsoft recognized the importance
of a CD-ROM cache and Windows 95/98 does CD-ROM caching
on its own, but there's certainly room for improvement here.
The cache that comes with Windows 95/98 has a maximum size
of a little over 1MB. That's it. Although this might be large
enough for some applications, others will certainly benefit
by a larger cache. If you have enough memory, CD-Quick Cache
can set a RAM cache as large as 32MB. If disk space is available,
the cache can be as large as 256MB! And unlike the Windows
95/98 CD-ROM cache, CD-Quick Cache's cache is persistent.
It remembers your CD-ROM data from CD to CD and also between
sessions! Another difference is the Windows 95/98 cache uses
fixed read ahead buffering. You can select the amount of read
ahead (or even turn it off), but once its set, it stays that
way. Some applications will benefit by doing read ahead while
others will suffer for it. The only way to optimize operation
for a wide variety of applications is by using dynamic read
ahead buffering as CD-Quick Cache does.
When CD-Quick Cache is loaded on your computer, every time
you read data from your CD-ROM drive CD-Quick Cache also saves
it to RAM or your (comparatively) faster hard drive. The next
time this same data is needed, CD-Quick Cache intercepts the
request and retrieves the data from its cache. The time to
access and move a block of CD-ROM data from RAM memory is
microseconds. Even if you use your hard drive for cache storage,
access time is measured in several milliseconds.
It's not enough to just store CD-ROM data in a fast cache.
It's very important to manage this data in an intelligent
manner. The reason caching
works is due to probability. It's likely that you'll need
the data you've just read some time in the future.
Most applications work this way, and as mentioned, it's the
basis for caching. It's certainly possible to write a program
that reads a CD-ROM's data only once from start to finish,
but it's not at all likely that any useful application would
ever work this way.
After the amount of data read is equal to the size of CD-Quick
Cache's cache, some data in the cache must be discarded to
make room for new data. CD-Quick Cache uses a "Least
Recently Used" (LRU) algorithm when it's time to decide
which data gets overwritten. As you might expect from the
name, with LRU the oldest data gets replaced first. CD-Quick
Cache's LRU scheme is very effective at keeping the data you'll
most likely want in the cache rather than on the CD.
If data is requested that's not in the cache, it's important
that CD-Quick Cache discover this quickly. CD-Quick Cache
uses a hash table to quickly locate data stored in its cache
An additional technique used by some caching software is
sector read ahead buffering. This system is also based on
probability. When reading data from a CD-ROM it's most likely
that the next block of data you'll want is the block following
the one you've just read. This certainly won't always be the
case. We're just going with the odds here, but the odds are
right often enough.
It's important here to note that you can have a cache that
does not do read ahead buffering. This is a separate feature
from the cache itself. If implemented properly, it's another
way to increase performance.
When a cache does read ahead buffering, it reads more data
than you've requested. The requested block of data is read
from the CD-ROM, but so is the next sector or two (or three).
The requested data is passed to the application that requested
it, and it's also stored in the cache along with the read
ahead data. When and if your application wants data from these
additional sectors, it's available in the cache.
So, doing read ahead buffering is a good thing right? Well
that all depends. If we're reading data sequentially, it's
another trick we can use to boost performance. But what if
we're reading data from the CD randomly? Reading a block,
jumping to a new location, reading some more, moving again,
reading again, etc. If we always read ahead the same amount,
a cache can quickly fill up with data you'll never want. This
can greatly reduce your effective cache size. If two out of
every three reads is wasted, you're using a cache that's actually
only one third the physical size. For example, say you've
got a 1.5 Mb cache set. If you're reading randomly and you
read two additional sectors for each sector read, your working
cache size is really only 512K. You're wasting 1 Mb of memory!
CD-Quick Cache does do read ahead buffering, but in an intelligent
manner. CD-Quick Cache's read ahead is adaptive. It constantly
monitors the actual pattern of your application's reads and
then dynamically adjusts the amount to read ahead. So if data
is being read sequentially (reading ahead is a good thing),
CD-Quick Cache reads ahead. If data is being read randomly
(reading ahead is a very bad thing), CD-Quick Cache doesn't
read ahead. The software actually uses fuzzy logic to determine
the degree of randomness and set the amount to read ahead.
This dynamic read ahead insures CD-Quick Cache will make the
very best use of your precious RAM (or disk space).
From phone directories to games, more and more CD-ROM applications
are requiring multiple CD sets. To deal with this situation,
CD-Quick Cache has the ability to track up to 256 different
CD-ROM titles. It does this by computing a unique signature
for each CD-ROM from a combination of its Volume Name and
Serial Number. This allows it to remember data when you change
discs. So, if you change CDs and then go back to an earlier
one, your data will still be able to be retrieved from the
cache. Volume titles are tracked on a most recently used basis,
so it is the last 256 CD-ROMs that are tracked.
In addition, if you use your hard drive for cache storage,
CD-Quick Cache has the ability to save your cached data and
volume information between sessions. This means you can shut
down your computer, come back to it days later, insert the
same CD-ROM that you used a few CD-ROMs back, and have it's
My Own Experience
A lot of the information from this article has been collected
in many places, but how did I like it? To be honest, with
some of my CDs, I didn't notice any improvements. This is
probably because there weren't very many reads needed. I do
notice an incredible difference when I use PrintMaster Platinum.
As with most desktop publishing suites, there are so many
graphics, that you get half a dozen CDs that contain all of
these graphics. You can browse a graphics catalog and then
you are prompted to insert a CD. I found that when I had CD-Quick
Cache running, this process was unbelievably fast, compared
to the way it was before. I could browse the graphics catalog
very quickly. My wife had been complaining that it would take
so long for each new page of graphics to show up, but not
Remember, I have nothing to do with this company and I am
getting nothing in return for having this article here (other
than a free copy of the program) so be sure to give the evaluation