(Please note that the feature described in LUCENE-2425 is required reading for this issue.)
The remoting split policy is an abstraction for a split index, where each sub-index points to a (potentially remote) URI-based directory. More formally, it establishes mappings between URIs and directories using the concept of a URIDirectory. Basically, that interface allows one to construct a directory instance based on a URI value.
Currently, the following two URIs have been mapped:
a) File URI: In this case, the path specified in the file URI maps to the location of a FSDirectory. While the path usually points to a folder in the local file system, it may refer to a NFS mount point, in which case the sub-directory is not co-located with the super-directory of the split index.
b) RAM URI: In this case, the path specified in the URI may be empty, in which case a volatile RAMDirectory is created. In the event the path is non-empty, the RAM-based directory acts as a facade for a file-system-based directory.
More (industrial-strength) URI mappings are currently in the works, and will be added as they become available.