SLAPD-MDB(5) File Formats Manual SLAPD-MDB(5)
NAME
slapd-mdb - Memory-Mapped DB backend to slapd
SYNOPSIS
/etc/ldap/slapd.conf
DESCRIPTION
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 uses a hierarchical database layout which supports sub-
tree renames.
CONFIGURATION
These slapd.conf options apply to the mdb backend. That is, they must
follow a "backend mdb" line and come before any subsequent "backend" or
"database" lines.
idlexp <exp>
Specify a power of 2 for the maximum size of an index slot. The
default is 16, yielding a maximum slot size of 2^16 or 65536.
Once set, this option applies to every mdb database instance.
The specified value must be in the range of 16-30.
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 argu-
ments 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.
dbnosync
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 particu-
lar, 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 committed.
directory <directory>
Specify the directory where the LMDB files containing this data-
base and associated indexes live. A separate directory must be
specified for each database. The default is /var/lib/ldap.
envflags {nosync,nometasync,writemap,mapasync,nordahead}
Specify flags for finer-grained control of the LMDB library's op-
eration.
nosync This is exactly the same as the dbnosync directive.
nometasync
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 no-
sync are set, the nosync flag takes precedence.
writemap
Use a writable memory map instead of just read-only. This
speeds up write operations but makes the database vulnera-
ble to corruption in case any bugs in slapd cause stray
writes into the mmap region.
mapasync
When using a writable memory map and performing flushes on
each commit, use an asynchronous flush instead of a syn-
chronous flush (the default). This option has no effect if
writemap has not been set. It also has no effect if nosync
is set.
nordahead
Turn off file readahead. Usually the OS performs readahead
on every read request. This usually boosts read perfor-
mance 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 Windows.
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 in-
dexes. If only an <attr> is given, the indices specified for de-
fault 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 at-
tribute.
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 nosub-
types may be specified to disallow use of this index by named
subtypes. Note: changing index settings in slapd.conf(5) re-
quires rebuilding indices, see slapindex(8); changing index set-
tings dynamically by LDAPModifying "cn=config" automatically
causes rebuilding of the indices online in a background task.
maxentrysize <bytes>
Specify the maximum size of an entry in bytes. Attempts to store
an entry larger than this size will be rejected with the error
LDAP_ADMINLIMIT_EXCEEDED. The default is 0, which is unlimited.
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 possi-
ble, (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.
multival {<attrlist>|default} <integer hi>,<integer lo>
Specify the number of values for which a multivalued attribute is
stored in a separate table. Normally entries are stored as a sin-
gle blob inside the database. When an entry gets very large or
contains attributes with a very large number of values, modifica-
tions on that entry may get very slow. Splitting the large at-
tributes out to a separate table can improve the performance of
modification operations. The threshold is specified as a pair of
integers. If the number of values exceeds the hi threshold the
values will be split out. If a modification deletes enough values
to bring an attribute below the lo threshold the values will be
removed from the separate table and merged back into the main en-
try blob. The threshold can be set for a specific list of at-
tributes, or the default can be configured for all other attrib-
utes. The default value for both hi and lo thresholds is
UINT_MAX, which keeps all attributes in the main blob.
rtxnsize <entries>
Specify the maximum number of entries to process in a single read
transaction when executing a large search. Long-lived read trans-
actions prevent old database pages from being reused in write
transactions, and so can cause significant growth of the database
file when there is heavy write traffic. This setting causes the
read transaction in large searches to be released and reacquired
after the given number of entries has been read, to give writers
the opportunity to reclaim old database pages. The default is
10000.
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 alloca-
tion. 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 im-
pact 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 used.
ACCESS CONTROL
The mdb backend honors access control semantics as indicated in
slapd.access(5).
FILES
/etc/ldap/slapd.conf
default slapd configuration file
SEE ALSO
slapd.conf(5), slapd-config(5), slapd(8), slapadd(8), slapcat(8),
slapindex(8), slapmodify(8), OpenLDAP LMDB documentation.
ACKNOWLEDGEMENTS
OpenLDAP Software is developed and maintained by The OpenLDAP Project
<http://www.openldap.org/>. OpenLDAP Software is derived from the Uni-
versity of Michigan LDAP 3.3 Release. Written by Howard Chu.
OpenLDAP 2.6.10+dfsg-1 2025/05/22 SLAPD-MDB(5)
Generated by dwww version 1.16 on Tue Dec 16 04:46:50 CET 2025.