Looking at the source of FSDirectory it seem easy enough to add a property to disable indexes.I would assume that removing this lock could increase performance especially to allow multiple concurrent searches on the same data.
There was talk about providing that in an upcoming version. Until then
you can try RODirectory:
http://www.csita.unige.it/software/free/lucene/
This way if you wanted to search a directory and you new it was 100% necessary to search it readonly you could go ahead and do it without having to write lock files.
I will write some unit tests to see what the performance is here. It should be trivial to create an index of a few hundred megs and then run about 200k queries across it with 30 threads or so... then disable locks and see what the total time spent was...
If it's substantial I think it makes sense to make this contribution :)
Kevin
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