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Volume Compression Operation
Disk volume compression works by setting up a virtual volume on your hard disk. In essence, a software-driven volume is created on the system and special drivers used to make this volume appear to be a physical hard disk. This isn't really that radical a concept, since many devices use software drivers to allow them to appear to the system under a drive letter.
When you decide to create a compressed volume on your disk, here is what the software that is creating it actually does, in approximate terms:
When you are using compression, then, what you see as a compressed volume is really just a giant file on a real hard disk. In some cases, you will be able to use both disks. For example, when I used to set up older systems with, say, 340 MB hard disks, I would often split the disk by creating a compressed drive called D: from say, 150 MB of space from C:. So then C: would have 190 MB free and a 150 MB compressed volume file. The compression drivers will create a logical D: volume from the 150 MB CVF. D: would typically be able to hold somewhere between 200 MB and 300 MB itself, depending on the compression ratio of the files.
Another option generally provided by the compression software is the ability to "substitute" the compressed volume in place of the host volume it is made from. This is normally done if you are creating a CVF that takes up an entire disk partition. Let's suppose you have a 540 MB hard disk that is partitioned into a 300 MB C: and a 240 MB D:, and you want to compress the entire D:. What the software will normally do after it creates the CVF taking up all of D: is to "map" the host D: drive to a much higher-up letter like H:, and then make the CVF appear as D: in its place. This allows the seamless compression of a hard disk while retaining its previous letter address.
Warning: I don't recommend
doing this with the C: boot partition, even though Microsoft's DOS DriveSpace program
sometimes recommends this by default. In my opinion it is better to create a separate
compressed volume and leave the boot volume C: alone, so that the system can be booted
more easily in the event of a problem with the compression.
Warning: If you delete the
compressed volume file from the host drive, guess what happens to your compressed volume?
Poof. Don't do it. :^)