As described in Indices an index is the data structure that powers GSS searches. It can be a fairly sizable data structure (see Index Size Calculation and this begs the question: where to store it? GSS provides two options: memory or file. For information on how to configure these properties see Configuration Properties
RAM (in-memory) directories must be reconstructed each time an application server is started (unless persistence is used, see Index Persistence. They are fast to access but their memory requirements may exceed the resources available. RAM directories may be very useful for testing however, as they do not hold state.
File indexes use the local file system to store the index.Even though the J2EE specification does not cover file system access in practice this works with all supported versions documented in a separate document, Curam Supported Prerequisites document. Naturally the better the performance of the underlying filesystem used the better the performance of GSS will be.