When Coolo looked into how to get rid of (Another) UnionFS for Live CDs and came up with the DoenerFS (now clicfs) idea, I remembered that my friend Arnd has workded on fake write support for cramfs. So I took his code and ported it to SquashFS to see how that goes. My expectation was that it might be faster than Coolo’s clicfs using FUSE. Here are some results using openSUSE-KDE4-LiveCD-i586-i686-Build0098 booting into runlevel 3:
- clicfs: 637MB ISO Image booting in 1:28 min (0:24 min from RAM)
- squashfs-rw: 751M ISO Image booting in 1:50 min (0:28 min from RAM)
The difference in the sizes of the ISO images are due to the fact that clicfs is using LZMA compression while SquashFS is still using the in-kernel GZIP implementation. Surprisingly the clicfs image isn’t only smaller but is also faster booting on real media and from RAM (using KVM). So even if we ignore the fact that clicfs is optimized for limiting the number of seeks on disk the SquashFS implementation is still slower. It would be interesting to see if it is just the LZMA compression that is making the difference or something completely different.
The patches for the SquashFS fake write support are here: http://git.infradead.org/users/jblunck/linux-2.6.git?a=shortlog;h=refs/heads/squashfs-rw-v1.