What’s behind “lzma compressed livecds”
There are various ways to build a live cd and since 11.2 Milestone 1 there is a new one: clicfs. I’ll try to explain:
The challenge with a live cd is the size of the CD and what you put on it, 700MB is not enough for a typical desktop experience. So most (if not all) distributions use compression to squeeze about 2GB on a CD. There are various compression file systems, most famous is squashfs - which is since 4.0 even in the kernel mainline (>= 2.6.29) and uses gzip compression, even though there exist patches to make it use lzma. These lzma patches are not very often refreshed and not officially supported by the squashfs authors. This might actually change soon as kernel 2.6.30 has lzma decompression built in - but it’s not there yet.. There is also cloop, which I think is exclusive to Knoppix. It’s also gzip compressed and has a different semantic than squashfs. More to that later.
But however you compress, it comes with a catch: your compression will create a read only file system. But you need to write to it, not to all places, but in various (/var/run, /var/tmp, /tmp/, often in /etc, surely in /home). For a long time, the live cds existent created one large tmpfs and symlinked all the places and files that needed writing. But this is pretty unflexible and also takes more memory than really necessary. The new solution was first unionfs, which has a long history with many ups and downs and then later aufs. Aufs is Another unionfs and is the reason for one of the downs of unionfs. With a union you can generate a file system that is actually a map of two: the read only part and a read write part. But neither of them has good short term prospect of getting in the kernel mainline (aufs is trying hard at the moment, but still my personal guess is: 11.2 will pass without it being in mainline).
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