Whist investigating an orthogonal logging issue I encountered a couple of differences between squid/varnish which I didn't know about:

* Varnish does not give subsecond request time information
* Varnish does give subsecond request processing time info
* Varnish calls it 'hit/200' or 'miss/302' instead of 'TCP_MEM_HIT/200' or 'TCP_MISS/302'
* Varnish does not URL encode the user agent field

Example log lines:
amssq41.esams.wikimedia.org 1013692039 2013-07-31T23:00:02.331 0 XXX TCP_MEM_HIT/200 614 GET http://meta.wikimedia.org/XXX NONE/- image/png http://en.wikipedia.org/XXX - Mozilla/5.0%20(Wind... en-US en;q=0.8 -
cp1006.eqiad.wmnet 1442176851 2013-07-31T23:00:02.452 0 XXX TCP_MISS/302 406 GET http://meta.wikimedia.org/XXX NONE/- - http://en.m.wikipedia.org/XXX - Mozilla/5.0%20(i.... en-us -
cp3012.esams.wikimedia.org 823553992 2013-07-31T23:00:02 0.000119448 XXX hit/200 20 GET http://meta.m.wikimedia.org/XXX - image/png http://de.m.wikipedia.org/XXX XXX Mozilla/5.0 (iPho... de-de -

~Matt Walker
Wikimedia Foundation
Fundraising Technology Team