[ccache] Compression on or off by default?

[Joel Rosdahl]

Hi,

Lars Gustäbel's compression patch (which will be incorporated in ccache 3.0)
enables compression by default, and if you don't want compression you have to
set CCACHE_NOCOMPRESS. I'm still a bit undecided about whether defaulting to
compression is a good idea, though. Maybe we should be more conservative here
and require CCACHE_COMPRESS to be set to enable compression instead? (Note that
the question only is about the default behaviour when storing files in the
cache -- ccache will still be able to read compressed and uncompressed files
from the cache regardless of the CCACHE_(NO)COMPRESS setting.)

The main argument I see for making compression opt-in is that hard-linking
doesn't work for compressed files (where "doesn't work" means that ccache will
fall back to copying), so if you would like to try out hard-linking, you must
set both CCACHE_NOCOMPRESS and CCACHE_HARDLINK, and also build up the cache
again. Or, if you currently have enabled hard-linking with ccache 2.4, you need
to take the explicit action of disabling compression after an upgrade to get
the previous behaviour.

Another argument is maybe that disk space is cheap nowadays, and most people
probably want to optimize for speed instead of disk space. On the other hand,
the overhead of using compression is very small. In fact, I am unable to
consistently measure any performance impact whatsoever. (Lars Gustäbel's own
measurements can be found at <http://gustaebel.de/lars/ccache/>.) And, by
compressing the cached files, more files will fit in the cache and also in the
OS disk cache.

Does anyone have an opinion to share about this?

-- Joel