Using LMDB


Introduction

Issue 4699 - PR 4716 (Aug 2021) implements an experimental feature allowing to replace Berkeley Database (bdb) by Open LDAP Lightning Memory-Mapped Database Manager (lmdb)

Pre-requisites

This work was integrated on Fedora master branch. (Not yet in REHL but planned for 9.1 or 9.2)

You now need to install both lmdb-libs and libdb to run 389ds ( and lmdb-devel and libdb-devel to build 389ds )

Basic configuration


In the configuration berkeley database is identified as “bdb” and Open LDAP Lightning Memory-Mapped Database Manager as “mdb” ( mdb is prefered instead of lmdb to avoid the confusion with ldbm (which is the 389ds module that implements the backend of the database (and that prefix some config parameters))

Checking which lib is active:

dsconf instance backend config get | grep nsslapd-backend-implement

    ( should be bdb or mdb )

Switch which lib is used by default

For CI tests and dscreate tool there are two way to proceed: - Setting environment variable: NSSLAPD_DB_LIB=mdb - Modifying lib389: DEFAULT_DB_LIB = ‘mdb’ in src/lib389/lib389/_constants.py:

Switch a single instance

- dsconf instance backend config set --db_lib bdb
- dsconf instance backend config set --db_lib mdb

Notes: - this require a server restarts - the first time the lib is changed, the db will be empty (so all backends should be exported in ldif from the old lib and reimported in the new lib) - If you want to revert, if all backends are replicated and changelog expiration is long enough, replication should be able to resync the database

Known impacts

There are a few behavior differences between bdb and mdb due to the architecture of both dbs - on line import/reindex does strongly impact ldap write operation performance (because mdb accepts only a single open write transaction at any time) - A range search on attribute with equality index become unindexed if of the the attribute value is longer than 507 bytes. ( the reason is that the value is hashed to comply with mdb key size limit, and hash fnction does not preserve lexicographic order)

mdb specific parameters

There are a few parameters specific to mdb visisble with: dsconf instance backend config get

- nsslapd-mdb-max-size Maximum database size (0 means it is computed from available disk space up to a value of 1GB). Value may be as great as the available disk space but beware that it is memory mmaped memory. So:
    - the file will tend to grow up to its maximum
    - if real memory is smaller than the file then pages will be swappped 
    - My first experience about configuring oversized database was interresting:
        at first I did not capped the value to 1Gb and the basic import scenario was spending
        hours before failing because disk was full (with a real db size around 10Gb on the test VM)
        while once capped to 1 GB the import was successful in less than 10 minutes 
	==> *Correct sizing matters*
- nsslapd-mdb-max-readers Maximum nomber of readers (note: maximum number of writer is 1 ) 
    The default value is 10 + number of working threads (and should probably not need any change)
- nsslapd-mdb-max-dbs Maximum number of files in the db 
    Set by default to 128,
    It may need to be increased ( should be greater than the total number of indexes and of backends)
Last modified on 23 August 2021