Index File Formats

[Jaly]

In version 2.1, the file format was changed to allow lock-less
commits (ie, no more commit lock). The change is fully backwards
compatible: you can open a pre-2.1 index for searching or adding/
deleting of docs. When the new segments file is saved (committed), it
will be written in the new file format (meaning no specific "upgrade"
process is needed). But note that once a commit has occurred, pre-2.1
Lucene will not be able to read the index.

In version 2.3, the file format was changed to allow segments to share
a single set of doc store (vectors & stored fields) files. This allows
for faster indexing in certain cases. The change is fully backwards
compatible (in the same way as the lock-less commits change in 2.1).

我们采用2.1版本的Lucene吧。