Most advanced compression software will give you some control over how much compression
you want to use on the volume, and even let you select certain types of files (based on
their extension) not to try to compress. In general, the more you try to compress the
volume, the more you can cram onto the disk, but the more advanced compression algorithms
can result in performance degradation because they work harder to squeeze the files more
so they can compress that little bit more. The degree of degradation depends on several
factors, but is especially dependent on the speed of the system you are using. A slower
processor can cause compression to result in a serious performance hit, while a faster
processor can in some cases make compression improve performance.
Improve performance through compression? How is that possible? Let's consider two
copies of an identical 100 MB file; one copy is uncompressed and the other is compressed
2:1 on a volume on the same hard disk. Suppose we need to scan every byte of each of these
files. We can read the uncompressed file at a faster rate per byte because we don't
have the overhead of decompression. However, we have to read twice as many bytes from the
disk, because the file is taking up more space on the disk. The compressed file is using
only 50 MB of physical disk sectors, instead of 100 MB.
As you probably know, hard disk access is much slower than the processor, memory,
chipset and other system components. For this reason, removing the need to read 50 MB of
data from the disk can save more time than the time required for the overhead of
decompression! If you have a fast processor that is spending a great deal of time waiting
for data from the slow hard disk then you actually get a performance boost with some types
of files by using compression.
The important factor is how fast the system is relative to the hard disk. If the
processor is fast and the hard disk slow, you will see this effect. If the processor is
slow and the hard disk fast, compression will cause a noticeable slowdown. This is why I
don't recommend compression on 486-class machines (unless hard disk space is a severe
problem), while I use it myself for two of my partitions on my Pentium-class machine. I
usually notice no slowdown in using these volumes.
There is "middle ground" as well in terms of the compression level. You can
usually choose how much compression to use on the volume, ranging from high compression to
none at all. Sometimes volumes are used with no compression just to get the benefits of
slack reduction that compression can provide.
Finally, there are some types of files that just don't belong on compressed disks. If
you have a bunch of large ZIP files, don't put them on a compressed volume, since there is
no benefit--they are already compressed and so volume compression will not help them at
all. The same thing applies to most multimedia files such as JPEG or GIF images--they have
already been compressed internally. Storing these files on compressed volumes will also
throw off your free space estimates because
they will take up much more space than a compressible file of the same size would.
Next: Memory Issues with Compression Drivers
