Compass supports RAMDirectory, but not InstantiatedIndex, a memory-only index that was added to Lucene more than a year ago. InstantiatedIndex offers better much better performance than RAMDirectory, but consumes more memory and is still considered experimental. See: http://lucene.apache.org/java/2_4_0/api/org/apache/lucene/store/instantiated/InstantiatedIndex.html https://issues.apache.org/jira/browse/LUCENE-550 Since InstantiatedIndex comes with Lucene 2.4.0, it should be available as a configuration option in the next Compass version.