Slack Reduction Using Volume Compression

Slack is the space wasted due to unused storage at the end of large clusters. When a great number of files are stored on a disk with a large cluster size, a lot of the disk is wasted due to overhead. One way to eliminate this is to use a utility program to create a ZIP file archive, for example, containing all the files. But this results in the files not being readily accessible.

Volume compression tools like DriveSpace 3 can in fact save space even when set not to compress any files at all, due to slack space reduction. Internally, DriveSpace 3 compressed drives allocate files on a sector-by-sector basis. This means that they have an effective cluster size of 512 bytes, because they use an internal format that lets them store more than one file in what would be a cluster on a regular drive.

Some people set up DriveSpace 3 volumes just to get this advantage. It can be configured so that no files are actually compressed (for performance reasons, to save on the overhead of decompression) but you still get the slack reduction because of the slack reduction feature. You still have some of the risks associated with using a compressed volume however, and also the loss of some memory for the compressed volume driver, which has to operate even if you set it to no compression (slack reduction only).

Next: Compression Level and Performance Considerations