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DouBle, by Jean-Marc Verbavatz, provides a compressed block device. You tell it the name of a block device or (fixed-size) regular file, some compression parameters (compression algorithm, cluster size, storage granularity), and a (conservative) expected compression ratio. It then provides a block device (say `/dev/dbl0') that has more blocks than the file or device where the data is actually being stored. If things don't compress as well as you expected, and it runs out of space, then you have a problem.
DouBle is a good choice if you need a block device for something other than putting an ext2 filesystem on. I would not recommend DouBle for storing a filesystem.
Anyway, a URL for DouBle (for Linux 1.2.5) is http://sunsite.unc.edu/pub/Linux/kernel/patches/diskdrives/double-0.4l1.tar.gz. This isn't current; the last known version was 0.49 (for Linux 2.0), which apparently can no longer be found on the Internet.
See section Why was e2compr written?, section e2compr with the loop driver, section Can I still use DouBle?, for other comments on DouBle.
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