One source of confusion in the use of compressed disk volumes relates to the amount of
free space reported on the drive. This can--and will--change in sometimes unexpected ways
as the compressed disk fills up, based on how compressible the files placed on the volume
are. The reason is simple: the free space on a real disk volume is easily determined
simply by examining how many clusters are free in the file allocation table, and
multiplying by the cluster size. With a compressed volume, we don't know how much space is
free until we know what will be put on the volume, because some files can be compressed a
great deal, and some not at all.
In fact, when you see a report of disk space free on a compressed volume, what you are
seeing is an estimate. Every compressed volume has a number associated with it
called its estimated compression ratio, which is what tells the compressed volume
driver how well you think the files on this volume are going to be compressed. This number
can be set on a volume-by-volume basis, and should be estimated, ideally, from the actual
compression ratio that the volume is using with its current files. Because it makes
their tools look impressive, many compression utilities default this estimate to something
like 2:1, even though the average disk volume in my experience will not compress at a
figure that high. Usually 1.6 to 1.8 is more typical (depending on the settings and of
course, to a great deal on what is stored on the drive).
Using the estimated compression ratio, the system will determine an estimated amount of
free space by multiplying the uncompressed free space by the estimated ratio. This is what
you see as free space on the drive. If you change the estimated compression ratio, the
report of free disk space changes as well; in reality, you have not changed at all the
capacity of the compressed volume.
As soon as you copy files to the compressed disk, they are compressed at whatever rate
the compression software can manage for the type of file. A huge text file could be
compressed at 3:1; a ZIP file--which is already compressed internally--will not compress
at all. The amount of space these real files takes up will vary, and so the amount of free
space will change, depending on what the files are.
OK, time for an example. 
Let's suppose we have an empty 500 MB hard disk that we
want to compress. We run DriveSpace 3, say, and it sets up a 500 MB CVF on the host disk,
and creates a new compressed drive called F: for example. When we look at F:, we see 1000
MB free. Why? Because the default is a 2:1 estimated compression ratio. This can be
changed, of course. For now, let's leave it at 2:1. Here's what the disk looks like:
Storage |
Used Space |
Free Space |
Total |
Uncompressed Total
(Inside the CVF) |
0 MB |
500 MB |
500 MB |
Compression Ratio |
-- |
2:1 |
2:1 |
Compressed Total |
0 MB |
1000 MB |
1000 MB |
So we have 500 MB of "true" space on the host disk, in the CVF, and 1000 MB
of space in the compressed disk, assuming our 2:1 ratio. Now suppose we copy to this empty
disk a 100 MB file that cannot be compressed very much; let's suppose it can only be
compressed at a ratio of 1.25 to 1. This means we will use up not 50 MB of real CVF space
as we would expect from a file compressible at 2:1, but rather 80 MB (100 divided by
1.25). Here's what the disk will look like now:
Storage |
Used Space |
Free Space |
Total |
Uncompressed Total
(Inside the CVF) |
80 MB |
420 MB |
500 MB |
Compression Ratio |
1.25:1 |
2:1 |
1.88:1 |
Compressed Total |
100 MB |
840 MB |
940 MB |
As you can see, the 500 MB total for the CVF always remains the same (unless you resize
the volume). But we have "lost" 60 MB from the compressed volume; it now has 840
MB free instead of the 900 MB we would expect after copying a 100 MB file to it. In
reality, all that happened is that we weren't able to compress the file at the 2:1 ratio
we expected.
Now let's copy another file to the compressed disk, let's say a 180 MB database file
that will compress at a ratio of 4 to 1. (This is quite common with large database files,
believe it or not.) This file will take only 45 MB of storage in the CVF, instead of the
90 MB that is "par" for a 2:1 volume. Here's what will happen:
Storage |
Used Space |
Free Space |
Total |
Uncompressed Total
(Inside the CVF) |
125 MB |
375 MB |
500 MB |
Compression Ratio |
2.24:1 |
2:1 |
2.06:1 |
Compressed Total |
280 MB |
750 MB |
1030 MB |
After storing this file our totals have increased, because it used much less space than
we "expected". In fact, our compressed volume is now "larger" than
when we started! While this is an extreme example, this shows why the free space on a
compressed disk tends to move around. The most common situation is when someone sets up a
1000 MB compressed disk and starts copying huge ZIP files to it. Guess what--ZIP files are
already compressed and cannot be compressed further. As soon as you copy about 500 MB of
ZIP files to a compressed volume of that size, it will be full.
Next: Slack Reduction Using Volume Compression
