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
operation.
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 trans-
action if the operating system crashes. If both nometa-
sync and nosync are set, the nosync flag takes prece-
dence.
writemap
Use a writable memory map instead of just read-only. This
speeds up write operations but makes the database vulner-
able 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
synchronous flush (the default). This option has no ef-
fect if writemap has not been set. It also has no effect
if nosync is set.
nordahead
Turn off file readahead. Usually the OS performs reada-
head on every read request. This usually boosts read per-
formance but can be harmful to random access read perfor-
mance 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
default are maintained. Note that setting a default does not
imply that all attributes will be indexed. Also, for best per-
formance, an eq index should always be configured for the ob-
jectClass attribute.
A number of special index parameters may be specified. The in-
dex type sub can be decomposed into subinitial, subany, and sub-
final indices. The special type nolang may be specified to dis-
allow 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.
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 pro-
cesses. 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
single blob inside the database. When an entry gets very large
or contains attributes with a very large number of values, modi-
fications on that entry may get very slow. Splitting the large
attributes 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 thresh-
old 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 entry blob. The threshold can be set for a spe-
cific list of attributes, or the default can be configured for
all other attributes. 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
transactions 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 evalua-
tion. 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 par-
ticular 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 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 2.5.13+dfsg-5 2022/07/14 SLAPD-MDB(5)
Generated by dwww version 1.15 on Tue Sep 2 04:31:51 CEST 2025.