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Free Disk Space And Disk Compression Utilitie
DocumentID: 613779
Revision Date: 29-Feb-96 1:36:15 PM

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

Problem

Solutions: Customers using disk compression utilities such as Stacker, SuperStor, and DoubleSpace may find that the amount of free disk space reported by DOS or their compression utility may be wrong. For example, during a test here at WordPerfect, a computer with MS-DOS 6.0 and DoubleSpace installed reported 20M of free disk space, but ran out of disk space after only 14M of WP 6.0 files were installed.

The reason this happens is very simple. First of all, remember that there are actually two different amounts of free disk space that are in play here: the actual amount of free, uncompressed disk space, and the amount of compressed data that can be stored in that uncompressed space. (Remember, it's the data that's compressed, not the space.)

The problem is that the compression utility doesn't know how much data will actually fit in that space. It varies dramatically depending on the kind of file being stored. Some files compress as much as 16:1 (16 to 1) meaning the file compresses to 1/16 its original size. Other files don't compress at all or compress very little. Files that compress well are text files, data files, most .EXE files, etc. Files that don't compress very well are .GIF files, .ZIP files, .ARC files, WP's .LEX and .THS files, and other files that are already using some form of compression in their data format.

The compression utility must make an educated guess as to how much it will be able to store in the available space. It does this based on a compression ratio formula. On a typical drive with a broad mix of file types, Stacker and SuperStor will average 1.8:1 or 1.9:1 compression ratio. DoubleSpace averages slightly lower, about 1.5:1 or 1.6:1. The problem is compounded by the fact that these utilities are optimistic in their expectations.

DoubleSpace defaults to an expected compression ration of 1.7:1. The user can change this number in the DBLSPACE utility, but the lowest allowable ratio is 1.6:1. The computer mentioned in the first paragraph, when changed to a 1.6:1 ratio, dropped from 20M to 15M free, still too high, but much closer to the actual amount in this case. Stacker and SuperStor also allow users to customize their expected compression ratios. But users still won't know what the correct ratio is until they actually try copying files to the drive. That is the only accurate way to find out. (Keep in mind that if the user had been copying only text files or WP documents to this hard drive, the user might have easily gotten 20M or more of files loaded before running out of space.)

When customers call and report problems such as running out of disk space during install when they had plenty of space before installing, try to help them understand how these compression utilities estimate space. They will need to free up more space before installing their WPCorp product.

Note: SuperStor is often misspelled SuperStore, Super Store, or Super Stor.

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