Ubuntu Manpage: slapd-mdb - Memory-Mapped DB backend to slapd

Provided by: slapd_2.5.18+dfsg-0ubuntu0.22.04.2_amd64

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 subtree 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
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.

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 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 committed.

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 /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 transaction if the operating system crashes. If both nometasync
and nosync 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 vulnerable 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 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 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 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 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.

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 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.

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, modifications 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 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 entry blob. The
threshold can be set for a specific 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 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 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

ACKNOWLEDGEMENTS

       OpenLDAP   Software   is   developed   and   maintained   by    The    OpenLDAP    Project
       <http://www.openldap.org/>.   OpenLDAP Software is derived from the University of Michigan
       LDAP 3.3 Release.  Written by Howard Chu.