4.5. Tuning the Database Cache for Searches

The database attributes that affect search performance mainly define the amount of memory available to the server. The maximum values that can be set for the database's cache size attributes depends on the amount of real memory on the machine. Roughly, the amount of available memory on the machine should always be greater than sum total of the default database cache size and sum of each entry cache size.
Use caution when changing these cache sizing attributes. The ability to improve server performance with these attributes depends on the size of the database, the amount of physical memory available on the machine, and whether directory searches are random (that is, if the directory clients are searching for random and widely scattered directory data).
If the database does not fit into memory and if searches are random, attempting to increase the values set on these attributes does not help directory performance. In fact, changing these attributes may harm overall performance.
The attributes of each database used to store directory data can be resized.
To improve the cache hit ratio on search operations, increase the amount of data that the Directory Server maintains in the database cache, as described in Section 6.5, “Setting the Database Cache Size”, by editing the values for the nsslapd-dbcachesize parameter.