Since early 2.x times Lucene used a skip/seek/write trick to patch the length of the terms dict into a place near the start of the output data file. This however made it impossible to use Lucene with append-only filesystems such as HDFS.
In the post-flex trunk the following code in StandardTermsDictWriter initiates this:
and completes this in close():
I propose to change this layout so that this pointer is stored simply at the end of the file. It's always 8 bytes long, and we known the final length of the file from Directory, so it's a single additional seek(length - 8) to read it, which is not much considering the benefits.