Linux VFS cache hit rate script v2

[Jake Maul]

Hello all,

Based on the list's feedback about the previous script (particularly
in relation to the "N/A" assumption), as well as a modified version
from Dave Wright, I have rewritten this script as attached. Also
included is a new 'probe end' block, inspired by 'entropo' from
Serverfault (http://serverfault.com/questions/157612/is-there-a-way-to-get-cache-hit-miss-ratios-for-block-devices-in-linux).
Note that this link is where I first started out from a couple weeks
ago.

This seems to offer much more repeatable and sane results, although
it's not without fault. For instance, it's possible for an
ioblock.request probe to register without a corresponding
vfs.read.return hit. Hence, there's a check in here for cache_bytes <
0, which should (in theory) not be necessary. Fortunately, in my
testing it appears to be a very small discrepancy, so I feel mostly
safe about just pretending it didn't happen- it seems to be dwarfed by
real I/O, and I probably wouldn't have noticed it if I wasn't testing
on a totally-idle server.

http://sourceware.org/systemtap/wiki/WSCacheHitRate is updated as
well, including props for those who've helped out in some way. :)

Thanks,
Jake
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