This was stolen from the Lucene mailing list as a strategy to warm up an IndexSearcher:
Something like dd if=/path/to/index/foo.cfs of=/dev/null
Basically, force the data through the kernel preemptively, so FS caches it.
Run vmstat while doing it, and if the index hasn't been cached by the FS, you should see a spike in IO activity while dd is running.
source: (something in the middle of the thread)