Wiping files on compressed disks.

[jpinson@fcdarwin.org.ec]

I did a few tests on wiping compressed (Stacker) files:

Sdir, the Stacker directory command, reported a 900k PKZip file 
had a compression ratio of 1.0:1  (no compression).

I wiped the file using the same character repeatedly, and sdir 
reported the resultant file had a compression ratio of 15.9:1

I wiped another copy of the zip file using sets of increasing 
characters (0-255).   After this wipe the compression ratio was 
8.0:1

Lastly, I wiped the file using random characters, generated using 
Turboc's random() function.

This time, the compression ratio was 1.0:1, the same as the 
original.

Sounds like wiping with random characters may indeed be the way 
to go to avoid "slack" at the end of the file.

One interesting note:   When I fragmented the original zip file 
into 50K segments with a "chop" program, sdir reported that each 
segment had a compression ratio of 1.1:1, even though the 
original file showed no compression.

When I created 10K segments, I got a compression ratio of 1.6:1

Pkzip however, was unable to compress these file segments at all.

I suspect that Stacker is not really compressing these smaller 
files in the normal sense, but is storing them more efficiently 
(better sector or cluster size?).

Jim Pinson