Currently the default max buffer size for MMapDirectory is 256MB on 32bit and Integer.MAX_VALUE on 64bit:
But, in windows on 64-bit, you are practically limited to 8TB. This can cause problems in extreme cases, such as: http://www.lucidimagination.com/search/document/7522ee54c46f9ca4/map_failed_at_getsearcher
Perhaps it would be good to change this default such that its 256MB on 32Bit OR windows, but leave it at Integer.MAX_VALUE
on other 64-bit and "64-bit" (48-bit) systems.