SLAPD-MDB(5)                  File Formats Manual                 SLAPD-MDB(5)

       slapd-mdb - Memory-Mapped DB backend to slapd


       The mdb backend to slapd(8) uses OpenLDAP's Lightning Memory-Mapped DB
       (LMDB) library to store data.  It relies completely on the underlying
       operating system for memory management and does no caching of its own.
       It is the recommended primary database backend.

       The mdb backend is similar to the hdb backend in that it uses a
       hierarchical database layout which supports subtree renames. It is both
       more space-efficient and more execution-efficient than the bdb backend,
       while being overall much simpler to manage.

       These slapd.conf options apply to the mdb backend database.  That is,
       they must follow a "database mdb" line and come before any subsequent
       "backend" or "database" lines.  Other database options are described in
       the slapd.conf(5) manual page.

       checkpoint <kbyte> <min>
              Specify the frequency for flushing the database disk buffers.
              This setting is only needed if the dbnosync option is used.  The
              checkpoint will occur if either <kbyte> data has been written or
              <min> minutes have passed since the last checkpoint.  Both
              arguments default to zero, in which case they are ignored. When
              the <min> argument is non-zero, an internal task will run every
              <min> minutes to perform the checkpoint.  Note: currently the
              <kbyte> setting is unimplemented.

              Specify that on-disk database contents should not be immediately
              synchronized with in memory changes.  Enabling this option may
              improve performance at the expense of data security. In
              particular, if the operating system crashes before changes are
              flushed, some number of transactions may be lost.  By default, a
              full data flush/sync is performed when each transaction is

       directory <directory>
              Specify the directory where the LMDB files containing this
              database and associated indexes live.  A separate directory must
              be specified for each database.  The default is

       envflags {nosync,nometasync,writemap,mapasync,nordahead}
              Specify flags for finer-grained control of the LMDB library's

              nosync This is exactly the same as the dbnosync directive.

                     Flush the data on a commit, but skip the sync of the meta
                     page. This mode is slightly faster than doing a full
                     sync, but can potentially lose the last committed
                     transaction if the operating system crashes. If both
                     nometasync and nosync are set, the nosync flag takes

                     Use a writable memory map instead of just read-only. This
                     speeds up write operations but makes the database
                     vulnerable to corruption in case any bugs in slapd cause
                     stray writes into the mmap region.

                     When using a writable memory map and performing flushes
                     on each commit, use an asynchronous flush instead of a
                     synchronous flush (the default). This option has no
                     effect if writemap has not been set. It also has no
                     effect if nosync is set.

                     Turn off file readahead. Usually the OS performs
                     readahead on every read request. This usually boosts read
                     performance but can be harmful to random access read
                     performance if the system's memory is full and the DB is
                     larger than RAM. This option is not implemented on

       index {<attrlist>|default} [pres,eq,approx,sub,<special>]
              Specify the indexes to maintain for the given attribute (or list
              of attributes).  Some attributes only support a subset of
              indexes.  If only an <attr> is given, the indices specified for
              default are maintained.  Note that setting a default does not
              imply that all attributes will be indexed. Also, for best
              performance, an eq index should always be configured for the
              objectClass attribute.

              A number of special index parameters may be specified.  The
              index type sub can be decomposed into subinitial, subany, and
              subfinal indices.  The special type nolang may be specified to
              disallow use of this index by language subtypes.  The special
              type nosubtypes may be specified to disallow use of this index
              by named subtypes.  Note: changing index settings in
              slapd.conf(5) requires rebuilding indices, see slapindex(8);
              changing index settings dynamically by LDAPModifying "cn=config"
              automatically causes rebuilding of the indices online in a
              background task.

       maxreaders <integer>
              Specify the maximum number of threads that may have concurrent
              read access to the database. Tools such as slapcat count as a
              single thread, in addition to threads in any active slapd
              processes. The default is 126.

       maxsize <bytes>
              Specify the maximum size of the database in bytes. A memory map
              of this size is allocated at startup time and the database will
              not be allowed to grow beyond this size. The default is 10485760
              bytes. This setting may be changed upward if the configured
              limit needs to be increased.

              Note: It is important to set this to as large a value as
              possible, (relative to anticipated growth of the actual data
              over time) since growing the size later may not be practical
              when the system is under heavy load.

       mode <integer>
              Specify the file protection mode that newly created database
              files should have.  The default is 0600.

       searchstack <depth>
              Specify the depth of the stack used for search filter
              evaluation.  Search filters are evaluated on a stack to
              accommodate nested AND / OR clauses. An individual stack is
              assigned to each server thread.  The depth of the stack
              determines how complex a filter can be evaluated without
              requiring any additional memory allocation. Filters that are
              nested deeper than the search stack depth will cause a separate
              stack to be allocated for that particular search operation.
              These allocations can have a major negative impact on server
              performance, but specifying too much stack will also consume a
              great deal of memory.  Each search stack uses 512K bytes per
              level. The default stack depth is 16, thus 8MB per thread is

       The mdb backend honors access control semantics as indicated in

              default slapd configuration file

       slapd.conf(5), slapd-config(5), slapd(8), slapadd(8), slapcat(8),
       slapindex(8), OpenLDAP LMDB documentation.

       OpenLDAP Software is developed and maintained by The OpenLDAP Project
       <>.  OpenLDAP Software is derived from
       University of Michigan LDAP 3.3 Release.  Written by Howard Chu.

OpenLDAP 2.4.40                   2014/09/20                      SLAPD-MDB(5)