Improve server performance on searches by tuning database settings. The database attributes that affect performance mainly define the amount of memory available to the server. There are two kinds of database caches, one for the default database cache and the other for the entry cache. The server has one default database cache per server, and one entry cache per database.

To improve the cache hit ratio on search operations, increase the amount of data that the Directory Server maintains in the database cache. Do this by increasing the cache size. The maximum values that can be set for these 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 two 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 following attributes can be tuned:

To configure the default database attributes that apply to all other database instances:

This displays the Database tabs in the right pane.

The tabs displayed in the right pane control parameter settings for this database.

Every Directory Server contains a transaction log which writes operations for all the databases it manages. Whenever a directory database operation such as a modify is performed, the server logs the operation to the transaction log. For best performance, the directory does not perform the operation immediately. Instead, the operation is stored in a temporary memory cache on the Directory Server until the operation is completed.

If the server experiences a failure, such as a power outage, and shuts down abnormally, the information about recent directory changes that were stored in the cache is lost. However, when the server restarts, the directory automatically detects the error condition and uses the database transaction log to recover the database.

Although database transaction logging and database recovery are automatic processes that require no intervention, it can be advisable to tune some of the database transaction logging attributes to optimize performance.

By default, the database transaction log file is stored in the /var/lib/dirsrv/slapd-instance_name/db directory along with the database files themselves. Because the purpose of the transaction log is to aid in the recovery of a directory database that was shut down abnormally, it is a good idea to store the database transaction log on a different disk from the one containing the directory database. Storing the database transaction log on a separate physical disk may also improve directory performance.

To change the location of the database transaction logfile, use the following procedure:

At regular intervals, the Directory Server writes operations logged in the transaction log to the disk and logs a checkpoint entry in the database transaction log. By indicating which changes have already been written to the directory, checkpoint entries indicate where to begin recovery from the transaction log, thus speeding up the recovery process.

By default, the Directory Server is set up to send a checkpoint entry to the database transaction log every 60 seconds. Increasing the checkpoint interval may increase the performance of directory write operations. However, increasing the checkpoint interval may also increase the amount of time required to recover directory databases after a disorderly shutdown and require more disk space due to large database transaction log files. Therefore, only modify this attribute if you are familiar with database optimization and can fully assess the effect of the change.

To modify the checkpoint interval while the server is running, use the ldapmodify^[18] command-line utility to add the nsslapd-db-checkpoint-interval attribute to the cn=config,cn=ldbm database,cn=plugins,cn=config entry.

For more information on the syntax of the nsslapd-db-checkpoint-interval attribute, refer to the Directory Server Configuration, Command, and File Reference. For instructions on using ldapmodify, see Section 2.2.4, “Adding and Modifying Entries Using ldapmodify”.

Durable transaction logging means that the temporary database transaction log is, in fact, physically written to disk.

When durable transaction logging is disabled, every directory database operation is written to the database transaction log file but may not be physically written to disk immediately. If a directory change was written to the logical database transaction log file but not physically written to disk at the time of a system crash, the change cannot be recovered. When durable transactions are disabled, the recovered database is consistent but does not reflect the results of any LDAP write operations that completed just before the system crash.

By default, durable database transaction logging is enabled. To disable durable transaction logging, use the following procedure:

To improve update performance when full transaction durability is not required, use the nsslapd-db-transaction-batch-val attribute to specify how many transactions will be batched before being committed to the transaction log. Setting this attribute to a value of greater than 0 causes the server to delay committing transactions until the number of queued transactions is equal to the attribute value. For transaction batching to be valid, the nsslapd-db-durable-transaction attribute must be set to on.

To specify or modify transaction batching while the server is running, use the ldapmodify^[18] command-line utility to add the nsslapd-db-transaction-batch-val attribute to the cn=config,cn=ldbm database,cn=plugins,cn=config entry.

For more information on the syntax and values of the nsslapd-db-transaction-batch-val attribute, refer to the Directory Server Configuration, Command, and File Reference. For instructions on using ldapmodify, see Section 2.2.4, “Adding and Modifying Entries Using ldapmodify”.