The LDBM backend back-ldbm used to be the standard general purpose data storage of slapd since UMich's slapd. It is built over a neutral interface to many Un*x storage systems, including dbm, gdbm, ndbm and Berkeley's Sleepycat. It is now obsoleted by back-bdb and back-hdb, which means that right now (2.2, transition toward 2.3) it's essentially feature-frozen and only major bugs get fixed. back-ldbm has been completely dropped from OpenLDAP 2.4. Users are strongly encouraged to upgrade to back-bdb or back-hdb.