Steps to open a file/handled by kernel?

[peter]

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Hi guys, im in discussion with one of my compsci mates to start an 
intresting little project, its not concrete yet, not at all, i really 
need to get a good grasp of what im going to have to code before i 
decide to jump in or not, ill give you a quick run down

Basically we want to make a sort of.. auto compression/uncompression of 
files as there accessed/not accessed, basically to save a hell of alot 
of space, but before we even BEGIN to do it, i really need to understand 
the calls and the "arguments" and the stages the kernel/userspace (if it 
is invovled (if userspace is even the right word!) takes to read from a 
file (and write to it while we are at it)

any information people feel is relevant or any sort of url u can point 
me to or just a little guide of waht happens would be appreciiated, 
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Cheers
:)
Peter