Hash index allows to perform index read operations for 1 (one) I/O operation, and index write for 3 (three) I/O operations as maximum. Hash index algorithm is based on extendible hashing Extendible Hashing algorithm. Hash index does not support range queries, but it's noticeable faster (about 2 times on 10M records) than SB-Tree index.
NOTE: There is an issue about enhancement of hash index to avoid slowdown introduced by random I/O operations using LSM Tree approaches: Issue #1757._