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Disk Compression Explained
DocumentID: 605108
Revision Date: 29-Feb-96 1:31:13 PM

The information in this document applies to:
WordPerfect® 5.1 for DOS

Problem

Symptoms: The customer is using SuperStor to compress a 512K RAM drive. After compression, approximately 960K is free in the RAM drive. The customer then loads WP{WP}.SPW, SP{WP}US.LEX, and WP{WP}US.SUP into the RAM drive. These files total about 377K; however, after loading them, there are zero bytes free in the RAM drive. Solutions: WP{WP}.SPW and SP{WP}US.LEX are already compressed files and SuperStor cannot compress them further. Therefore, they take up more space than the directory listing states. The free space indicated on a compressed drive is an estimate based on a standard compression ratio. This ratio was incorrect for these two files.

How/Why:
A compressed drive is interpreted as a drive with more available space than it had before compression. This is an illusion. A 10M hard drive, once compressed, may show 20M free. But, in reality, there is only 10M free. Under normal circumstances, 20M of data may be stored on this 10M of free space. When compressed files (e.g., WP's .SPW and .LEX files, .ZIP files, or .GIF files) are stored, they cannot be compressed much further, if at all. So a 1M .ZIP file will actually occupy 1M of uncompressed space, into which 2M of compressible data could have been stored. It would still appear as a 1M file in the directory, but the free space would now be around 18M, not 19M. This can be confusing to customers who have just begun using compression utilities.

Answer:

Details:


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