I would say for the first release of a new feature, default it to off. That 
gives people a chance to 'play' with it without potentially breaking existing 
installations that are simply upgrading. If there are no major problems with 
the feature in that first release then make it a default in the next release 
after that (e.g. ccache-3.1).

-----Original Message-----
From: ccache-boun...@lists.samba.org [mailto:ccache-boun...@lists.samba.org] On 
Behalf Of Joel Rosdahl
Sent: 02 March 2010 22:11
To: ccache@lists.samba.org
Subject: [ccache] Compression on or off by default?

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
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