If this is IDLP you don't need to store full keys, just distance from base start. This alone saves up to 20 bytes. However this works only if the max distance is ensured to never be reached.

Also I'm willing to bet an arm and leg that mmap-ing the DP file will lose big time against an actual database file, when there are lots of DPs, since they'll be spread around the disk, I wonder what your OS will have to say about that, when seeking around and pages start to swap in and out (besides all running apps stalling due to low free memory)

But seems that there's some kind of competition on these forums on who can reinvent a rounder wheel.