While looking at LUCENE-4123, i thought about this:
I don't like how mmap creates a separate mapping for each CFS slice, to me this is way too many mmapings.
Instead I think its slicer should map the .CFS file, and then when asked for an offset+length slice of that, it should be using .duplicate()d buffers of that single master mapping.
then when you close the .CFS it closes that one mapping.
this is probably too scary for 4.0, we should take our time, but I think we should do it.