It appears that there are several versions of both DoubleSpace and DriveSpace. Information about each version as I
find get it will be posted in this answer.
DoubleSpace uses MRCI, which uses "a variant of Lempel-Ziv encoding". A non-technical explanation of the DoubleSpace compression algorithm:
DoubleSpace first identifies the repeated sequences, and then writes them as <offset, length>. Offset is the number of bytes to the left of where the match starts [...] For example:
The rain in Spain falls mainly on the plain.
This sentence would be compressed as follows:
The rain <3,3>Sp<9,4>falls m<11,3>ly on <34,37>pl<15,3>.
The key is to analyze the frequency of occurrence of the offset and length values, and then to choose very short encoding [sic] for the most common values. – Source
The DoubleSpace naming convention for CVFs (Compressed Volume Files) is
nnn is a three-digit, right-aligned (
0-padded) number. If the DoubleSpace CVF was created from uncompressed data the extension will be
000; if the CVF was created empty the extension will be another number.
Windows 95 "DriveSpace"
This program, whilst producing files in the DoubleSpace format, seems to be having an identity crisis:
I still refer to it as 'win95 doublespace' though M$ call it 'drivespace'. – Source
It supports compressed drives of up to 512MB with a cluster size of 8KB.[1/FAQ]
This program supports compressed drives of up to 2GB with a cluster size of 512KB.[1/FAQ] It uses the SQ-0-0 compression scheme.[1/FAQ]
Thanks to misha256 for the link to dmsdos. Thanks to Tommy for the link to PC Mag.
Thanks to Tommy (here) and Jules (here) for identifying the DoubleSpace compression algorithm.
Thanks to manassehkatz (here), Tommy (here) and redsPL (here) for pointing out various errors.