E2compr 0.4 User Manual – Why

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When I (Antoine) got my laptop and installed Linux on it, it was prettyobvious that the hard drive was too small (240MB) to get a completeinstallation of all the tools I needed, while still having a DOSpartition (games for my children) and some space for users. Buying anew drive would have been quite expensive and not very practical for alaptop.

That’s why I immediately looked for tools that could do on-the-flycompression and decompression. I installed zlibc and tcx, and usedthem for a while. Though these are nice tools, there are somelimitations. For instance, you can’t write to compressed filesthrough zlibc; and tcx works only for executables. Of course theseare minor problems. These tools are doing what they were written for.I used them for some months, and one day I reinstalled some packageson my system: I realised that each time I reinstalled something, Ihad to compress by hand the manuals, the binaries if I wanted to….I found this very painful, and I started to dream about a file systemthat would do it in a transparent way. What I was looking for was asystem that could provide write access to compressed files, and forwhich I wouldn’t need to compress newly installed files myself.

Someone alerted me to the existence of DouBle by Jean-Marc Verbavatz(http://perso.cybercable.fr/~verbavat/double/). I got itand looked at how it did its work. There was however one thing thatmade me feel a little insecure: administrative data is compressed aswell. This means that if your system crashes you may lose much morethan a few blocks. Maybe I am a little over-anxious, but I couldn’tbear the idea of my son pressing the power off button while I wasworking. I felt this was a major problem for me. I was only ready touse DouBle instead of zlibc for files I could easily recover. [pjm: I canconfirm that a filesystem compressed with DouBle can run into bigproblems if the machine is powered off in the middle of a write. If youlose the first DouBle cluster of the partition then you might as wellsay goodbye to the whole filesystem. Unlike e2compr, there wasn’t evenany error recovery means except to read the source code until you can workout the file format and then use a raw disk editor.]

I then started to think about what I would really like to have on mymachine. The result is the e2compr patch for which you are nowreading the documentation. The ideal tools would meet the followingrequirements:

  1. automatic compression
  2. automatic decompression
  3. file access in both read and write modes
  4. flexible configuration
  5. secure

From the 3 tools mentioned above, DouBle is probably the best one. Itmeets requirements 1, 2, 3 and 4. Unfortunately I can’t live withthe idea that inodes are blindly compressed. Both tcx and zlibc meetrequirements 2 and 5. I don’t consider they are very flexible becausethere is no way to tune them. Note also that they will both fail todecompress data on a completely full file system.

E2compr obviously meets requirements 1, 2, 3, 4 and 5. Andthat’s why I wrote it.

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