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 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.
CONFIGURATION¶
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.
- 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.
- 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
SEE ALSO¶
slapd.conf(5),
slapd-config(5),
slapd(8),
slapadd(8),
slapcat(8),
slapindex(8), OpenLDAP LMDB
documentation.
ACKNOWLEDGEMENTS¶
OpenLDAP Software is developed and maintained by The OpenLDAP Project
<
http://www.openldap.org/>.
OpenLDAP Software is derived from
University of Michigan LDAP 3.3 Release. Written by Howard Chu.