The store module is one of the modules that we usually don't pay much attention to when configuring our cluster; however, it is very important. It is an abstraction between the I/O subsystem and Apache Lucene itself. All the operations that Lucene do with the hard disk drive are done using the store module. Most of the store types in Elasticsearch are mapped to an appropriate Apache Lucene directory class (http://lucene.apache.org/core/6_2_0/core/org/apache/lucene/store/Directory.html). The directory is used to access all the files the index is built of, so it is crucial to properly configure it.