Friday, October 26, 2012

Those strange zlib files

If you look at an OpenIndiana live CD, you'll see a couple of strangely named files - solaris.zlib and solarismisc.zlib. What are these, how can you access their contents, and how can you build your own?

These are actually archives of parts of the filesystem. In the case of solaris.zlib, it's /usr; for solarismisc.zlib it's /etc, /var, and /opt.

These are regular iso images, compressed with lofiadm. So you can use lofiadm against the file to create a lofi device, and mount that up as you would anything else. During the live boot, solaris.zlib gets mounted up at /usr. (There have to be some minimal bits of /usr in the boot archive; that's another story.)

Building these archives is very easy. Just go to where the directory you want to archive is and use mkisofs, like so:

mkisofs -o solaris.zlib -quiet -N \
    -l -R -U -allow-multidot \

    -no-iso-translate -cache-inodes \
    -d -D -V "compress" usr


Then you can ask lofiadm to compress the archive

lofiadm -C gzip solaris.zlib

The compression options are gzip, gzip-9, and lzma.  On the content I'm using, I found that basic gzip gives me 3-4x compression, and lzma a bit more at 4-5x. However, using lzma takes an order of magnitude longer to compress, and you get maybe half the read performance. There's a trade-off of additional space saving against the performance hit, which is clearly something you need to consider.

For Tribblix, I use the same trick, although I'm planning to get rid of the extra solarismisc.zlib. And I'm hoping that being lean will mean that I don't need the extra lzma compression, so I can stay fast.

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