Performance Tuning Guide

Red Hat Directory Server 9

Updated for Directory Server 9.1.2

Marc Muehlfeld

Red Hat Customer Content Services

Petr Bokoč

Red Hat Customer Content Services

Ella Deon Ballard

Red Hat Customer Content Services

Abstract

Tips for improving server and database performance.
This documentation is no longer maintained. For details, see Section 1, “Deprecated Documentation”.

1. Deprecated Documentation

Important

Note that as of June 10, 2017, the support for Red Hat Directory Server 9 has ended. For details, see “Red Hat Directory Server Life Cycle policy”. Red Hat recommends users of Directory Server 9 to update to the latest version.
Due to the end of the maintenance phase of this product, this documentation is no longer updated. Use it only as a reference!

2. Intro to Directory Server Performance Tuning

This article provides some procedures and options that administrators can use to optimize the performance of their Red Hat Directory Server deployments. Performance tuning a Directory Server instance is unique to each server because of differences for every server in its machine environment, directory size and data type, load and network use, even the types of operations that users and clients perform.
The purpose of this article is to highlight the features that Red Hat Directory Server provides for tracking and assessing server and database performance. There are also some procedures given to help tune server performance. For more in-depth planning information, however, check out the Red Hat Directory Server Deployment Guide, and for exhaustive command-line and UI-based administrative instructions, see the Red Hat Directory Server Administrator's Guide.
The performance tuning features here apply to Red Hat Directory Server 9.0.

2.1. Setting Goals for Directory Server Performance

Performance tuning is simply a way to identify potential (or real) bottlenecks in the normal operating environment of the server and then taking steps to mitigate those bottlenecks.
The general plan for performance tuning is:
  1. Assess the environment. Look at everything around the Directory Server: its usage, the load, the network connection and reliability, most common operations, the physical machine its on, along with any services competing for its resources.
  2. Measure the current Directory Server performance and establish baselines.
  3. Identify the server areas which can be improved.
  4. Make any changes to the Directory Server settings and, potentially, to the host machine.
  5. Measure the Directory Server performance again to see how the changes affected the performance.
Directory Server provides some sort of monitoring in three areas:
  • The server process (counters and logs)
  • The databases (counters)
  • Any database links (counters)
In the Directory Server, most performance measurements are going to be how well the Directory Server retrieves and delivers information to clients. With that in mind, these are the server areas that can be tuned for the best Directory Server performance (and these are the areas covered in this article):
  • Search operations
  • Indexing performance (which affects both search and write operations)
  • Database transactions
  • Database and entry cache settings
  • Database links
Other changes can be made to the host machine's settings or hardware which can also affect Directory Server performance:
  • Available memory (based on directory size)
  • Other servers running on the same machine (which could compete for resources)
  • Distributing user databases across other Directory Server instances on other machines
  • Balancing server loads due to network performance
These changes relate much more to planning an effective Directory Server deployment than changes that can be made to an instance. Reviewing the Deployment Guide can provide more detail about how to plan an optimal enterprise deployment.

2.2. Giving Feedback

If there is any error in this Performance and Tuning Guide or there is any way to improve the documentation, please let us know. Bugs can be filed against the documentation for Red Hat Directory Server through Bugzilla, http://bugzilla.redhat.com/bugzilla. Make the bug report as specific as possible, so we can be more effective in correcting any issues:
  1. Select the Red Hat Directory Server product.
  2. Set the component to Doc - performance-and-tuning.
  3. Set the version number to 9.0.
  4. For errors, give the page number (for the PDF) or URL (for the HTML), and give a succinct description of the problem, such as incorrect procedure or typo.
    For enhancements, put in what information needs to be added and why.
  5. Give a clear title for the bug. For example, "Incorrect command example for setup script options" is better than "Bad example".
We appreciate receiving any feedback — requests for new sections, corrections, improvements, enhancements, even new ways of delivering the documentation or new styles of docs. You are welcome to contact Red Hat Content Services directly at docs@redhat.com.

3. Tracking Server and Database Performance

Red Hat Directory Server has two methods of recording and tracking performance data: performance counters and logs. Counters are used to determine how well the Directory Server performing, particularly in database performance; logs are used to diagnose any problem areas with server and LDAP operations and configuration.
Performance counters focus on the operations and information of the Directory Server for the server, all configured databases, and database links (chaining databases).
There are three types of logs: access (for client connections), errors (for errors, warnings, and details of events), and audit (changes to Directory Server configuration). The access and error logs run by default (and the errors log is required for the server to run). Audit logging, because of the overhead, must be enabled manually.

Note

The access log is buffered. This allows full access logging even with highly loaded servers, but there is a time lag between when the event occurs in the server and when the event is written to the log.

3.1. Monitoring Server Activity

The Directory Server's current activities can be monitored from either the Directory Server Console or the command line. It is also possible to monitor the activity of the caches for all of the database.

Note

Some of the counters for Directory Server database attributes monitored by server use 64-bit integers, even on 32-bit systems (total connections, operations initiated, operations completed, entries sent, and bytes sent). On high-volume systems, this keeps the counters from rolling too quickly and skewing monitoring data.

3.1.1. Monitoring the Server from the Directory Server Console

  1. In the Directory Server Console, select the Status tab.
  2. In the navigation tree, select Performance Counters.
    The Status tab in the right pane displays current information about server activity. If the server is currently not running, this tab will not provide performance monitoring information.
The General Information table shows basic information about the server, which helps set a baseline about the statistics that have been gathered.

Table 1. General Information (Server)

Field Description
Server Version Identifies the current server version.
Startup Time on Server The date and time the server was started.
Current Time on Server The current date and time on the server.
The Resource Summary table shows the totals of all operations performed by that instance.

Table 2. Resource Summary

Resource Usage Since Startup Average Per Minute
Connections The total number of connections to this server since server startup. Average number of connections per minute since server startup.
Operations Initiated The total number of operations initiated since server startup. Operations include any client requests for server action,such as searches, adds, and modifies. Often, multiple operations are initiated for each connection. Average number of operations per minute since server startup.
Operations Completed The total number of operations completed by the server since server startup. Average number of operations per minute since server startup.
Entries Sent to Clients The total number of entries sent to clients since server startup. Entries are sent to clients as the result of search requests. Average number of entries sent to clients per minute since server startup.
Bytes Sent to Clients The total number of bytes sent to clients since server startup. Average number of bytes sent to clients per minute since server startup.
The Current Resource Usage table shows the current demands on the server.

Table 3. Current Resource Usage

Resource Current Total
Active Threads The current number of active threads used for handling requests. Additional threads may be created by internal server tasks, such as replication or chaining.
Open Connections The total number of open connections. Each connection can account for multiple operations, and therefore multiple threads.
Remaining Available Connections The total number of remaining connections that the server can concurrently open. This number is based on the number of currently open connections and the total number of concurrent connections that the server is allowed to open. In most cases, the latter value is determined by the operating system and is expressed as the number of file descriptors available to a task.
Threads Waiting to Write to Client The total number of threads waiting to write to the client. Threads may not be immediately written when the server must pause while sending data to a client. Reasons for a pause include a slow network, a slow client, or an extremely large amount of information being sent to the client.
Threads Waiting to Read from Client The total number of threads waiting to read from the client. Threads may not be immediately read if the server starts to receive a request from the client, and then the transmission of that request is halted for some reason. Generally, threads waiting to read are an indication of a slow network or client.
Databases in Use The total number of databases being serviced by the server.
The Connection Status table simply lists the current active connections, with related connection information.

Table 4. Connection Status

Table Header Description
Time Opened The time on the server when the connection was initially opened.
Started The number of operations initiated by this connection.
Completed The number of operations completed by the server for this connection.
Bound as The distinguished name used by the client to bind to the server. If the client has not authenticated to the server, the server displays not bound in this field.
Read/Write Indicates whether the server is currently blocked for read or write access to the client. There are two possible values:
Not blocked means that the server is idle,actively sending data to the client, or actively reading data from the client.
Blocked means that the server is trying to send data to the client or read data from the client but cannot. The probable cause is a slow network or client.
The Global Database Cache table lists the cache information for all databases within the Directory Server instance.

Note

Although the performance counter for the global database cache is listed with the other server performance counters in the Directory Server Console, the actual database cache entries are located and monitored in cn=monitor,cn=database_instance,cn=ldbm database,cn=plugins,cn=config, as are the other database activities.

Table 5. Global Database Cache Information

Table Header Description
Hits The number of times the server could process a request by obtaining data from the cache rather than by going to the disk.
Tries The total number of database accesses since server startup.
Hit Ratio The ratio of cache tries to successful cache hits. The closer this number is to 100%, the better.
Pages Read In The number of pages read from disk into the cache.
Pages Written Out The number of pages written from the cache back to disk.
Read-Only Page Evicts The number of read-only pages discarded from the cache to make room for new pages. Pages discarded from the cache have to be written to disk, possibly affecting server performance. The lower the number of page evicts the better.
Read-Write Page Evicts The number of read-write pages discarded from the cache to make room for new pages. This value differs from Pages Written Out in that these are discarded read-write pages that have not been modified. Pages discarded from the cache have to be written to disk, possibly affecting server performance. The lower the number of page evicts, the better.

3.1.2. Monitoring the Directory Server from the Command Line

The Directory Server's current activities can be monitored using LDAP tools such as ldapsearch, with the following characteristics:
  • Search with the attribute filter objectClass=*.
  • Use the search base cn=monitor; the monitoring attributes for the server are found in the cn=monitor entry.
  • Use the search scope base.
For example: 
ldapsearch -D "cn=directory manager" -W -p 389 -h server.example.com -x -s base -b "cn=monitor" "(objectclass=*)"
The monitoring attributes for the Directory Server are found in the cn=monitor entry.

Table 6. Server Monitoring Attributes

Attribute Description
version Identifies the directory's current version number.
threads The current number of active threads used for handling requests. Additional threads may be created by internal server tasks, such as replication or chaining.
connection:fd:opentime:opsinitiated:opscompleted:binddn:[rw] Provides the following summary information for each open connection (only available if you bind to the directory as Directory Manager):
fd — The file descriptor used for this connection.
opentime — The time this connection was opened.
opsinitiated — The number of operations initiated by this connection.
opscompleted — The number of operations completed.
binddn — The distinguished name used by this connection to connect to the directory.
rw — The field shown if the connection is blocked for read or write.
By default, this information is available to Directory Manager. However, the ACI associated with this information can be edited to allow others to access the information.
currentconnections Identifies the number of connections currently in service by the directory.
totalconnections Identifies the number of connections handled by the directory since it started.
dtablesize Shows the number of file descriptors available to the directory. Each connection requires one file descriptor: one for every open index, one for log file management, and one for ns-slapd itself. Essentially, this value shows how many additional condncurrent connections can be serviced by the directory. For more information on file descriptors, see the operating system documentation.
readwaiters Identifies the number of threads waiting to read data from a client.
opsinitiated Identifies the number of operations the server has initiated since it started.
opscompleted Identifies the number of operations the server has completed since it started.
entriessent Identifies the number of entries sent to clients since the server started.
bytessent Identifies the number of bytes sent to clients since the server started.
currenttime Identifies the time when this snapshot of the server was taken. The time is displayed in Greenwich Mean Time (GMT) in UTC format.
starttime Identifies the time when the server started. The time is displayed in Greenwich Mean Time (GMT) in UTC format.
nbackends Identifies the number of back ends (databases) the server services.
backendmonitordn Identifies the DN of each directory database.

3.2. Monitoring Database Activity

Note

Some of the counters for Directory Server database attributes monitored by server use 64-bit integers, even on 32-bit systems (entry cache hits, entry cache tries, the current cache size, and the maximum cache size). On high-volume systems, this keeps the counters from rolling too quickly and skewing monitoring data.

3.2.1. Monitoring Database Activity from the Directory Server Console

  1. In the Directory Server Console, select the Status tab.
  2. In the navigation tree, expand the Performance Counters folder, and select the database to monitor.
    The tab displays current information about database activity. If the server is currently not running, this tab will not provide performance monitoring information.
The Summary Information section shows the cumulative information for all of the databases being monitored and some cache-related configuration settings which are applied to all databases.

Table 7. Summary Information

Performance Metric Current Total
Read-Only Status Shows whether the database is currently in read-only mode. The database is in read-only mode when the nsslapd-readonly attribute is set to on.
Entry Cache Hits The total number of successful entry cache lookups. That is, the total number of times the server could process a search request by obtaining data from the cache rather than by going to disk.
Entry Cache Tries The total number of entry cache lookups since the directory was last started. That is, the total number of entries requested since server startup.
Entry Cache Hit Ratio
Ratio that indicates the number of entry cache tries to successful entry cache lookups. This number is based on the total lookups and hits since the directory was last started. The closer this value is to 100%, the better. Whenever an operation attempts to find an entry that is not present in the entry cache, the directory has to perform a disk access to obtain the entry. Thus, as this ratio drops towards zero, the number of disk accesses increases, and directory search performance drops.
To improve this ratio, increase the size of the entry cache by increasing the value of the nsslapd-cachememsize attribute in the cn=database_name, cn=ldbm database,cn=plugins,cn=config entry for the database. In the Directory Server Console, this is set in the Memory available for cache field in the database settings.
Current Entry Cache Size (in Bytes) The total size of directory entries currently present in the entry cache.
Maximum Entry Cache Size (in Bytes)
The size of the entry cache maintained by the directory.
This value is managed by the nsslapd-cachememsize attribute in the cn=database_name, cn=ldbm database,cn=plugins,cn=config entry for the database. This is set in the Memory available for cache field in the database settings in the Directory Server Console.
Current Entry Cache Size (in Entries) The number of directory entries currently present in the entry cache.
Maximum Entry Cache Size (in Entries)
DEPRECATED.
The maximum number of directory entries that can be maintained in the entry cache.
Do not attempt to manage the cache size by setting a maximum number of allowed entries. This can make it difficult for the host to allocate RAM effectively. Manage the cache size by setting the amount of RAM available to the cache, using the nsslapd-cachememsize attribute.
There are many different databases listed for the database monitoring page, by default, because databases are maintained for both entries and indexed attributes. All databases, though, have the same kind of cache information monitored in the counters.

Table 8. Database Cache Information

Performance Metric Current Total
Hits The number of times the database cache successfully supplied a requested page.
Tries The number of times the database cache was asked for a page.
Hit Ratio
The ratio of database cache hits to database cache tries. The closer this value is to 100%, the better. Whenever a directory operation attempts to find a portion of the database that is not present in the database cache, the directory has to perform a disk access to obtain the appropriate database page. Thus, as this ratio drops towards zero, the number of disk accesses increases, and directory performance drops.
To improve this ratio, increase the amount of data that the directory maintains in the database cache by increasing the value of the nsslapd-dbcachesize attribute. This is the Maximum Cache Size database setting in the Directory Server Console.
Pages Read In The number of pages read from disk into the database cache.
Pages Written Out The number of pages written from the cache back to disk. A database page is written to disk whenever a read-write page has been modified and then subsequently deleted from the cache. Pages are deleted from the database cache when the cache is full and a directory operation requires a database page that is not currently stored in cache.
Read-Only Page Evicts The number of read-only pages discarded from the cache to make room for new pages.
Read-Write Page Evicts The number of read-write pages discarded from the cache to make room for new pages. This value differs from Pages Written Out in that these are discarded read-write pages that have not been modified.

Table 9. Database File-Specific

Performance Metric Current Total
Cache Hits The number of times that a search result resulted in a cache hit on this specific file. That is, a client performs a search that requires data from this file, and the directory obtains the required data from the cache.
Cache Misses The number of times that a search result failed to hit the cache on this specific file. That is, a search that required data from this file was performed, and the required data could not be found in the cache.
Pages Read In The number of pages brought to the cache from this file.
Pages Written Out The number of pages for this file written from cache to disk.

3.2.2. Monitoring Database Activity from the Command Line

A database's current activities can be monitored using LDAP tools such as ldapsearch. The search targets the monitoring subtree of the LDBM database entry, cn=monitor,cn=database_name,cn=ldbm database,cn=plugins,cn=config. This contains all of the monitoring attributes for the that specific database instance.
For example: 
ldapsearch -D "cn=directory manager" -W -p 389 -h server.example.com -x -s base -b "cn=monitor,cn=database_name,cn=ldbm database,cn=plugins,cn=config" "(objectclass=*)"

Table 10. Database Monitoring Attributes

Attribute Description
database Identifies the type of database currently being monitored.
readonly Indicates whether the database is in read-only mode; 0 means that the server is not in read-only mode, 1 means that it is in read-only mode.
entrycachehits The total number of successful entry cache lookups. That is, the total number of times the server could process a search request by obtaining data from the cache rather than by going to disk.
entrycachetries The total number of entry cache lookups since the directory was last started. That is, the total number of search operations performed against the server since server startup.
entrycachehitratio
Ratio that indicates the number of entry cache tries to successful entry cache lookups. This number is based on the total lookups and hits since the directory was last started. The closer this value is to 100%, the better. Whenever a search operation attempts to find an entry that is not present in the entry cache, the directory has to perform a disk access to obtain the entry. Thus, as this ratio drops towards zero, the number of disk accesses increases, and directory search performance drops.
To improve this ratio, increase the size of the entry cache by increasing the value of the nsslapd-cachememsize attribute in the cn=database_name, cn=ldbm database,cn=plugins,cn=config entry for the database. In the Directory Server Console, this is set in the Memory available for cache field in the database settings.
currententrycachesize
The total size, in bytes, of directory entries currently present in the entry cache.
To increase the size of the entries which can be present in the cache, increase the value of the nsslapd-cachememsize attribute in the cn=database_name, cn=ldbm database,cn=plugins,cn=config entry for the database. In the Directory Server Console, this is set in the Memory available for cache field in the database settings.
maxentrycachesize
The maximum size, in bytes, of directory entries that can be maintained in the entry cache.
To increase the size of the entries which can be present in the cache, increase the value of the nsslapd-cachememsize attribute in the cn=database_name, cn=ldbm database,cn=plugins,cn=config entry for the database. In the Directory Server Console, this is set in the Memory available for cache field in the database settings.
dbcachehits The number of times the server could process a request by obtaining data from the cache rather than by going to the disk.
dbcachetries The total number of database accesses since server startup.
dbcachehitratio The ratio of cache tries to successful cache hits. The closer this number is to 100%, the better.
dbcachepagein The number of pages read from disk into the cache.
dbcachepageout The number of pages written from the cache back to disk.
dbcacheroevict The number of read-only pages discarded from the cache to make room for new pages. Pages discarded from the cache have to be written to disk, possibly affecting server performance. The lower the number of page evicts the better.
dbcacherwevict The number of read-write pages discarded from the cache to make room for new pages. This value differs from Pages Written Out in that these are discarded read-write pages that have not been modified. Pages discarded from the cache have to be written to disk, possibly affecting server performance. The lower the number of page evicts the better.
dbfilename-number The name of the file. number provides a sequential integer identifier (starting at 0) for the file. All associated statistics for the file are given this same numerical identifier.
dbfilecachehit-number The number of times that a search result resulted in a cache hit on this specific file. That is, a client performs a search that requires data from this file, and the directory obtains the required data from the cache.
dbfilecachemiss-number The number of times that a search result failed to hit the cache on this specific file. That is, a search that required data from this file was performed, and the required data could not be found in the cache.
dbfilepagein-number The number of pages brought to the cache from this file.
dbfilepageout-number The number of pages for this file written from cache to disk.
currentdncachesize
The total size, in bytes, of DNs currently present in the DN cache.
To increase the size of the entries which can be present in the DN cache, increase the value of the nsslapd-dncachememsize attribute in the cn=database_name, cn=ldbm database,cn=plugins,cn=config entry for the database.
maxdncachesize
The maximum size, in bytes, of DNs that can be maintained in the DN cache.
To increase the size of the entries which can be present in the cache, increase the value of the nsslapd-dncachememsize attribute in the cn=database_name, cn=ldbm database,cn=plugins,cn=config entry for the database.
currentdncachecount The number of DNs currently present in the DN cache.

3.4. Monitoring the Local Disk for Graceful Shutdown

When the disk space available on a system becomes too small, the Directory Server process (slapd) crashes. Any abrupt shutdown runs the risk of corrupting the database or losing directory data.
It is possible to monitor the disk space available to the slapd process. A disk monitoring thread is enabled using the nsslapd-disk-monitoring configuration attribute. This creates a monitoring thread that wakes every ten (10) seconds to check for available disk space in certain areas.
If the disk space approaches a defined threshold, then the slapd begins a series of steps (by default) to reduce the amount of disk space it is consuming:
  • Verbose logging is disabled.
  • Access logging and error logging are disabled.
  • Rotated (archived) logs are deleted.

Note

Error log messages are always recorded, even when other changes are made to the logging configuration.
If the available disk space continues to drop to half of the configured threshold, then the slapd begins a graceful shut down process (within a grace period); and if the available disk space ever drops to 4KB, then the slapd process shuts down immediately. If the disk space is freed up, then the shutdown process is aborted, and all of the previously disabled log settings are re-enabled.
By default, the monitoring thread checks the configuration, transaction log, and database directories. An additional attribute (nsslapd-disk-monitoring-logging-critical) can be set to include the logs directory when evaluating disk space.
Disk monitoring is disabled by default, but it can be enabled and configured by adding the appropriate configuration attributes to the cn=config entry. Table 12, “Disk Monitoring Configuration Attributes” lists all of the configuration options.
  1. Using ldapmodify, add the disk monitoring attributes. At a minimum, turn on the nsslapd-disk-monitoring attribute to enable disk monitoring. The default threshold is 2MB; this can be configured (optionally) in the nsslapd-disk-monitoring-threshold attribute.
    For example: 
    [jsmith@server ~]$ ldapmodify -D "cn=directory manager" -W -p 389 -x
dn: cn=config
changetype: modify
add: nsslapd-disk-monitoring
nsslapd-disk-monitoring: on
-
add: nsslapd-disk-monitoring-threshold 
nsslapd-disk-monitoring-threshold: 3000000
-
add: nsslapd-disk-monitoring-grace-period
nsslapd-disk-monitoring-grace-period: 20
  2. Restart the Directory Server to load the new configuration. 
    [root@server ~]# service dirsrv restart

Table 12. Disk Monitoring Configuration Attributes

Configuration Attribute Description
nsslapd-disk-monitoring Enabled disk monitoring. This is the only required attribute, since the other configuration options have usable defaults.
nsslapd-disk-monitoring-grace-period Sets a grace period to wait before shutting down the server after it hits half of the disk space limit. This gives an administrator time to address the situation. The default value is 60 (minutes).
nsslapd-disk-monitoring-logging-critical Sets whether to shut down the server if the log directories pass the halfway point set in the disk space limit.This prevents the monitoring thread from disabling audit or access logging or from deleting rotated logfiles.
nsslapd-disk-monitoring-threshold Sets the amount of disk space, in bytes, to use to evaluate whether the server has enough available disk space. Once the space reaches half of this threshold, then the server begins a shut down process. The default value is 2000000 (2MB).

3.5. Viewing Log Files

Note

The access and error logs are enabled by default and can be viewed immediately. before the audit log can be viewed, audit logging must be enabled for the directory, or the audit log will not be kept.
  1. In the Directory Server Console, select the Status tab.
  2. In the navigation tree, expand the Log folder. There are three folders available, for the access, error, and audit logs.
  3. When you select the log type to view, a table displays a list of the last 25 entries in the selected log.
  4. Optionally, change the settings of the log display and click Refresh to update the display.
    • The Select Log pull-down menu allows you to select an archived (rotated) log rather than the currently active log.
    • The Lines to show text box changes the number of log entries to display in the window.
    • The Show only lines containing text box sets a filter, including regular expressions, to use to display only certain matching log entries.

Note

Selecting the Continuous check box refreshes the log display automatically every ten seconds.= Continuous log refresh does not work well with log files over 10 megabytes.

3.6. Replacing Log Files with a Named Pipe

Many administrators want to do some special configuration or operation with logging data, like configuring an access log to record only certain events. This is not possible using the standard Directory Server log file configuration attributes, but it is possible by sending the log data to a named pipe, and then using another script or plug-in to process the data. Using a named pipe for the log simplifies these special tasks, like:
  • Logging certain events, like failed bind attempts or connections from specific users or IP addresses
  • Logging entries which match a specific regular expression pattern
  • Keeping the log to a certain length (logging only the last number of lines)
  • Sending a notification, such as an email, when an event occurs
The basic format of the script is is:

ds-logpipe.py named_pipe [ --user pipe_user ] [ --maxlines number ] [[ --serverpidfile file.pid ] | [ --serverpid PID ]] [ --servertimeout seconds ] [ --plugin=/path/to/plugin.py | [ pluginfile.arg=value ]]

More detailed usage information is in the Configuration, Command, and File Reference.
However, while that has the advantage of being simple to implement and not requiring any Directory Server configuration changes, simply running the script has a big disadvantage: all of the log viewers in the Directory Server Console and any script or tool (such as logconv.pl) that expect to access a real file will fail.
If the Directory Server instance will permanently use the named pipe rather than a real file for logging, then it is possible to reconfigure the instance to create the named pipe and use it for logging (as it does by default for the log files). When the Directory Server instance is configured to use the named pipe then all of the log analysis tools — the Directory Server Console and any Directory Server scripts — work fine.
Three things need to be configured for the log configuration for the instance:
  • The log file to use has to be changed to the pipe (nsslapd-*log)
  • Buffering should be disabled because the script already buffers the log entries (nsslapd-*log-logbuffering)
  • Log rotation should be disabled so that the server does not attempt to rotate the named pipe (nsslapd-*log-maxlogsperdir, nsslapd-*log-logexpirationtime, and nsslapd-*log-logrotationtime)
These configuration changes can be made in the Directory Server Console or using ldapmodify.
For example, this switches the access log to access.pipe: 
ldapmodify -D "cn=directory manager" -W -p 389 -h server.example.com -x

dn: cn=config
changetype: modify
replace: nsslapd-accesslog
nsslapd-accesslog: /var/log/dirsrv/slapd-instance_name/access.pipe
-
replace: nsslapd-accesslog-logbuffering
nsslapd-accesslog-logbuffering: off
-
replace: nsslapd-accesslog-maxlogsperdir
nsslapd-accesslog-maxlogsperdir: 1
- 
replace: nsslapd-accesslog-logexpirationtime
nsslapd-accesslog-logexpirationtime: -1
- 
replace: nsslapd-accesslog-logrotationtime
nsslapd-accesslog-logexpirationtime: -1

Note

Making these changes using the -f option will cause the server to close the current log file and switch to the named pipe immediately. This can be very helpful for debugging a running server and sifting the log output for specific messages.

3.7. Improving Logging Performance

Larger server deployments can generate several dozen of megabytes of logs per hour. Depending on the resources available on the server host machine, reconfiguring or disabling access logging can improve system and Directory Server performance.
Before disabling access logging, first configure access log buffering. Buffering writes all log entries directly to the disk, so that the Directory Server performance does not degrade even under a heavy load.
The access log is buffered by default, but make sure the log is using buffering for best performance. 
ldapmodify -D "cn=directory manager" -W -p 389 -x

dn: cn=config
changetype: modify
replace: nsslapd-accesslog-logbuffering
nsslapd-accesslog-logbuffering: on
If that does not improve performance, then disable access logging entirely. 
ldapmodify -D "cn=directory manager" -W -p 389 -x

dn: cn=config
changetype: modify
replace: nsslapd-accesslog-enabled
nsslapd-accesslog-enabled: off

Warning

Access logging is extremely helpful for debugging issues in the server and monitoring client connections and failed connection attempts. Don't disable access logging as the normal operating environment.
For alternatives, see Section 3.6, “Replacing Log Files with a Named Pipe”, since using named pipe log scripts can improve performance while still logging information on high performance production servers.

4. Optimizing System Performance

There are a limited number of areas where settings on the machine on which the Directory Server instance is running can be changed to improve Directory Server performance. Basically, the machine can be configured to grant the Directory Server more system resources (TCP ports and file descriptors) which helps it handle higher loads with better performance. The host machine can also be configured to allow better client access to the server (firewalls and DNS), which does not have a direct impact on Directory Server performance, but does affect using the server.

Note

The Red Hat Enterprise Linux knowledgebase includes a reference architecture (PDF) for setting up Red Hat Enterprise Linux to improve Oracle database performance. Similar adjustments can be made to any Red Hat Enterprise Linux system hosting a Red Hat Directory Server instance, and that performance tuning whitepaper is a good resource to help with Directory Server system tuning.

4.1. Tuning and Sizing System Memory

There are several different areas of the Red Hat Enterprise Linux machine that can be modified to improve the performance of the Directory Server itself.

Note

The cache settings here relate to the cache allowed to the Directory Server process, not to any cache settings in the Directory Server databases.

4.1.1. Setting the Minimum System Page Cache

Although Red Hat Enterprise Linux sets the page cache size dynamically, it is possible to require a minimum number of free pages by setting the vm.min_free_kbytes parameter in the system configuration. For example: 
# echo vm.min_free_kbytes=1024 >> /etc/sysctl.conf

4.1.2. Managing the Swap Space

Long-running processes (and Directory Server is normally a long-running process) do not use all of the pages that they obtain. Creating a swap space to partition the processes can improve the system's performance because the system memory is used for disk cache rather than page cache.
The /proc/sys/vm/swappiness kernel parameter sets a threshold for when processes should be swapped out in favor of I/O caching. The value of this is a percentage; the higher the percentage, the higher the I/O cache and the faster that pages are swapped. The default value is 60. This can be changed by editing the system configuration as root: 
# echo vm.swappiness=10 >> /etc/sysctl.conf
Constant swapping must be avoided. If swap usage is high or increasing, then increase the RAM (ideally) or the swap size.

4.1.3. Tuning for 32-Bit Systems

Ideally, Red Hat Directory Server will be run on 64-bit systems which naturally have the address space to handle very large databases and operations. Still, 32-bit systems can be optimized to the best possible performance.
First, use the hugemem kernel for databases that are 8GB or larger.
Next, configure huge pages on the system by setting how many large memory pages should be allocated and kept in RAM for shared memory:
  1. Check the amount of memory available for huge pages. 
      grep Hugepagesize /proc/meminfo
Hugepagesize:     4096 kB
    This means that each huge page is 4GB.
  2. Set the number of huge pages that can be used through the nr_hugepages kernel parameter. 
    # echo "vm.nr_hugepages=512" >> /etc/sysctl.conf
  3. Check the new huge page settings.
    The defined value for huge pages is viewable in Hugepagesize, while the total number of huge pages (set in nr_hugepages) is shown in HugePages_Total, both in the /proc/meminfo: 
    HugePages_Total:     512
HugePages_Free:      512
HugePages_Rsvd:      0
Hugepagesize:     4096 kB
  4. For the Directory Server databases to be able to use huge pages, add or set the memlock parameter for the Directory Server user (nobody by default) to a high enough seting to allow them to lock that much memory. 
    vim /etc/security/limits.conf

nobody        soft      memlock           1048576
nobody        hard      memlock           1048576

    Important

    Red Hat strongly recommends to change the default Directory Server user values and to create a dirsrv:dirsrv user instead of using the default nobody:nobody user.

4.2. Using dsktune for Preliminary Tuning

When a Directory Server instance is first set up, the setup command automatically runs a tool called dsktune which checks the current machine settings and compares them to the requirements for Directory Server.
This tool can be run again at any point to check whether any system settings should be adjusted. 
[jsmith@example ~]$ dsktune
Red Hat Directory Server system tuning analysis version 10-AUGUST-2007.

NOTICE : System is i686-unknown-linux2.6.18-164.2.1.el5 (2 processors).


NOTICE : The net.ipv4.tcp_keepalive_time is set to 7200000 milliseconds
(120 minutes).  This may cause temporary server congestion from lost
client connections.

WARNING: There are only 1024 file descriptors (hard limit) available, which
limit the number of simultaneous connections.  

WARNING: There are only 1024 file descriptors (soft limit) available, which
limit the number of simultaneous connections.

4.3. Tuning File Descriptors

The Directory Server is multi-threaded and efficiently handles many simultaneous requests and large numbers of connections.
Most of the time, the only limit on Directory Server threads is whatever limit the operating system imposes on the number of file descriptors available in the whole system and per process. The Directory Server uses file descriptors in a range of directory operations — logging, chaining, replication, client connections, and indexes. Adjusting the number of file descriptors for the Directory Server, then, can improve how well it handles large loads for a variety of operations.

4.3.1. Resetting the Host Machine's File Descriptors

For details, see the corresponding section in the Directory Server Installation Guide.

4.3.2. Setting Directory Server File Descriptor Values

For performance, the most relevant attribute is the nsslapd-maxdescriptors attribute. This tells the Directory Server how many file descriptors it has available. (This value cannot be larger than the system setting, or the server ignores it.)
Reserve file descriptors rarely need to be reset. Decreasing the number of reserve file descriptors can increase the number of LDAP clients that can connect to the server. However, too few reserve file descriptors can still hurt directory performance by causing server errors. There are three instances that can require raising the numbr of reserve file descriptors:
  • The server is replicating to a large number of consumer servers (more than 10), or the server is maintaining a large number of index files (more than 30).
  • The server is servicing a large number of LDAP connections.
  • There are error messages reporting that the server is unable to open file descriptors (the actual error message differs depending on the operation that the server is attempting to perform), but these error messages are not related to managing client LDAP connections.
To calculate the number of reserved file descriptors to set, use the following formula: 
nsslapd-reservedescriptors = 20 + (NldbmBackends * 4) + NglobalIndex +
ReplicationDescriptor + ChainingBackendDescriptors + PTADescriptors + SSLDescriptors
  • NldbmBackends is the number of ldbm databases.
  • NglobalIndex is the total number of configured indexes for all databases including system indexes. (By default 8 system indexes and 17 additional indexes per database).
  • ReplicationDescriptor is eight (8) plus the number of replicas in the server that can act as a supplier or hub (NSupplierReplica).
  • ChainingBackendDescriptors is NchainingBackend times the nsOperationConnectionsLimit (a chaining or database link configuration attribute; 10 by default).
  • PTADescriptors is 3 if PTA is configured and 0 if PTA is not configured.
  • SSLDescriptors is 5 (4 files + 1 listensocket) if SSL is configured and 0 if SSL is not configured.
To reset the file descriptors for Directory Server:
  1. Edit the /etc/sysconfig/dirsrv file and add a new line setting the process ulimit. 
    ulimit -n 8192
  2. Update the nsslapd-maxdescriptors and nsslapd-reservedescriptors values. 
    ldapmodify -D "cn=directory manager" -W -p 389 -h server.example.com -x

dn: cn=config
changetype: modify
replace: nsslapd-maxdescriptors
nsslapd-maxdescriptors: 8192
-
replace: nsslapd-reservedescriptors
nsslapd-reservedescriptors: 40
-
  3. Restart the Directory Server. 
    service dirsrv restart

4.3.3. Setting the Socket Connection Backlog

The listen service sets the number of sockets available to receive incoming connections. The backlog setting sets a maximum length for how long the queue for the socket (sockfd) can grow before refusing connections.
By default, the server sets this value to 128, but this can be tuned for your environment using the nsslapd-listen-backlog-size attribute. 
ldapmodify -D "cn=directory manager" -W -p 389 -x
				
dn: cn=config
changetype: modify
replace: nsslapd-listen-backlog-size
nsslapd-listen-backlog-size: 256

4.4. Setting Shell Limits for the Directory Server User

The Directory Server process runs as a regular user on the host system. By default, that user is nobody. However, Red Hat strongly recommends to use a different user and group name such as dirsrv during the installation. Once system settings have been changed, then the limits for that user can be updated.

Note

The Red Hat Enterprise Linux knowledgebase includes a reference architecture (PDF) for setting up Red Hat Enterprise Linux to improve Oracle database performance. Similar adjustments can be made to any Red Hat Enterprise Linux system hosting a Red Hat Directory Server instance, and that performance tuning whitepaper is a good resource to help with Directory Server system tuning.

4.4.1. Setting Shell Limits for File Descriptors

  1. As root, open the system's /etc/security/limits.conf file.
  2. Add two lines that set the hard and soft limits for the file descriptors (nofile) for the Directory Server user. By default, that user is nobody. However, Red Hat strongly recommends to use a different user and group name such as dirsrv during the installation. The soft limit sets how many file descriptors the user has available by default; the user can manually adjust that setting until they hit the hard limit. 
    nobody               soft        nofile          4096
nobody               hard        nofile          63536

    Important

    Do not set the hard limit for the Directory Server user equal to (or higher than) the maximum number of file descriptors assigned to the system itself in /proc/sys/fs/file-max. If the hard limit is too high and the user users all of those file descriptors, then the entire system runs out of file descriptors.

4.4.2. Setting Shell Limits for the Max Number of Processes

  1. As root, open the system's /etc/security/limits.conf file.
  2. Add two lines that set the hard and soft limits for the number of processes (nproc) for the Directory Server user. By default, that user is nobody. However, Red Hat strongly recommends to use a different user and group name such as dirsrv during the installation. The soft limit sets how many processes the user has available by default; the user can manually adjust that setting until they hit the hard limit. 
    nobody      soft      nproc      2047
nobody      hard      nproc      16384

    Important

    Do not set the hard limit for the Directory Server user equal to (or higher than) the maximum number of file descriptors assigned to the system itself in /proc/sys/fs/file-max. If the hard limit is too high and the user users all of those file descriptors, then the entire system runs out of file descriptors.

4.5. Tuning TCP Ports

If there is high traffic on a server — like the same machine is running both a web server and a Directory Server or there are a lot of clients who access the Directory Server directly — then it may be necessary to broaden the range of TCP ports available on the machine. 
echo "1024 65000" > /proc/sys/net/ipv4/ip_local_port_range

4.6. Changing Firewall Settings for the Directory Server Console

When remote clients are access the Directory Server instance, particularly the Directory Server Console or applications like Red Hat Certificate System, then make sure that the iptables configuration allows those connections past the firewall.
For example, if the instance is using the default ports and TLS/SSL is configured, then there are three ports that need to be opened in iptables.
  1. Open the iptables file. 
    # vi /etc/sysconfig/iptables
  2. Add a line to the configuration for each port that is in use for the Directory Server: 
    -A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 389 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 636 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 9830 -j ACCEPT

    Note

    Only add the TLS/SSL port (636) if TLS/SSL is actually enabled for the server.
  3. Restart the firewall: 
    # service iptables restart

4.7. DNS Requirements

It is very important that DNS and reverse DNS be working correctly on the host machine, especially if you are using TLS/SSL or Kerberos with Directory Server.
Configure the DNS resolver and the NIS domain name by the modifying the /etc/resolv.conf, /etc/nsswitch.conf, and /etc/netconfig files, and set the DNS resolver for name resolution.
Edit the /etc/defaultdomain file to include the NIS domain name. This ensures that the fully-qualified host and domain names used for the Directory Server resolve to a valid IP address and that that IP address resolves back to the correct host name.
Reboot the machine to apply these changes.

5. Tuning the Number of Locks

The lock mechanisms control how many copies of the Directory Server process can be running at one. For example, if there is an import job, then a lock is placed in the imports/ directory to prevent any other ns-slapd (normal), ldif2db (another import), or db2ldif (export) operations from running. If the server is running as normal, there is a lock in the server/ directory, which prevents import operations (but not export operations), while if there is an export operation, the lock in the exports/ directory allows normal server operations but prevents import operations.
If there are error messages indicating that the lock table is out of available locks (for example, libdb: Lock table is out of available locks), double the value of the nsslapd-db-locks attribute in the cn=config,cn=ldbm database,cn=plugins,cn=config entry.
For example, if the current value is 10000, set it to 20000. If the problem persists, double the number again. To monitor the current and maximum number of locks, do a search on cn=database,cn=monitor,cn=ldbm database,cn=plugins,cn=config. For example: 
ldapsearch -D "cn=directory manager" -W -p 389 -h server.example.com -x -sub -b "cn=database,cn=monitor,cn=ldbm database,cn=plugins,cn=config" objectclass=* | grep -- -locks: )
For more information on using LDAP utilities, see the Directory Server Administrator's Guide.

6. Improving Search Performance (and Balancing Read Performance)

The most effective way to improve search operations against the directory is to configure thorough indexes for entries, combined with reasonable limits on search results.

6.1. Using Indexes

An index (as it implies) is a tag that shows that a certain entry contains a certain attribute, without having to contain any other detail about the entry (which saves space and makes returning search results faster). Each index is organized around a Directory Server attribute and a certain way of matching that attribute:
  • Presence index (pres) simply shows what entries contain an attribute.
  • Equality index (eq) shows which attribute values match a specific search string.
  • Approximate index (approx) is used for efficient sounds-like searches, which shows entries which have a value that phonetically matches a string.
  • Substring index (sub) matches any substring of an attribute value to the given search string. (This index if very expensive for the server to maintain.)
  • International index uses a matching rule to match strings in a directory which contains values in languages other than English.
  • Browsing index, or virtual list view (VLV) index, sets an index to use to display entries in the Directory Server Console.

Note

Indexing is described in much more detail in the Administrator's Guide.
However, just creating indexes is not ipso facto going to increase server performance. Maintaining indexes puts a burden on the Directory Server for every modify, add, and delete operation by having to verify every attribute in the change against every index maintained by the server:
  1. The Directory Server receives an add or modify operation.
  2. The Directory Server examines the indexing attributes to determine whether an index is maintained for the attribute values.
  3. If the created attribute values are indexed, then the Directory Server generates the new index entries.
  4. Once the server completes the indexing, the actual attribute values are created according to the client request.
For example, the Directory Server adds the entry: 
dn: cn=John Doe, ou=People,dc=example,dc=com
objectclass: top
objectClass: person
objectClass: orgperson
objectClass: inetorgperson
cn: John Doe
cn: John
sn: Doe
ou: Manufacturing
ou: people
telephoneNumber: 408 555 8834
description: Manufacturing lead for the Z238 line of widgets.
The Directory Server is maintaining the following indexes:
  • Equality, approximate, and substring indexes for cn (common name) and sn (surname) attributes.
  • Equality and substring indexes for the telephone number attribute.
  • Substring indexes for the description attribute.
When adding that entry to the directory, the Directory Server must perform these steps:
  1. Create the cn equality index entry for John and John Doe.
  2. Create the appropriate cn approximate index entries for John and John Doe.
  3. Create the appropriate cn substring index entries for John and John Doe.
  4. Create the sn equality index entry for Doe.
  5. Create the appropriate sn approximate index entry for Doe.
  6. Create the appropriate sn substring index entries for Doe.
  7. Create the telephone number equality index entry for 408 555 8834.
  8. Create the appropriate telephone number substring index entries for 408 555 8834.
  9. Create the appropriate description substring index entries for Manufacturing lead for the Z238 line of widgets. A large number of substring entries are generated for this string.
Before creating new indexes, make sure to balance the overhead of maintaining the indexes against the potential improvements in search performance. Especially important, match the types of indexes that you maintain to the type of information stored in the directory and the type of information users routinely search for.
  • Approximate indexes are not efficient for attributes commonly containing numbers, such as telephone numbers.
  • Substring indexes do not work for binary attributes.
  • Equality indexes should be avoided if the value is big (such as attributes intended to contain photographs or passwords containing encrypted data).
  • Maintaining indexes for attributes not commonly used in a search increases overhead without improving global searching performance.
  • Attributes that are not indexed can still be specified in search requests, although the search performance may be degraded significantly, depending on the type of search.
  • The more indexes you maintain, the more disk space you require.

Note

Creating indexes is much more effective for directories which have a high search operation load and low modify operation load.

6.2. Tuning Directory Server Resource Settings

The server's performance can be managed and improved by limiting the amount of resources the server uses to process client search requests, which is done by defining four settings:
  • The maximum number of entries the server returns to the client in response to a search operation (size limit attribute).
  • The maximum amount of real time (in seconds) for the server to spend performing a search request (time limit attribute).
  • The time (in seconds) during which the server maintains an idle connection before terminating it (idle timeout attribute).
  • The maximum number of file descriptors available to the Directory Server (max number of file descriptors attribute).
To configure Directory Server to optimize performance:
  1. In the Directory Server Console, select the Configuration tab, and then select the topmost entry in the navigation tree in the left pane.
  2. Select the Performance tab in the right pane.
  3. Set the maximum number of entries the server will return to the client in response to a search operation by entering a new value in the Size Limit text box.
    To keep from setting a limit, type -1 in this text box.
  4. Enter the maximum amount of real time (in seconds) for the server to spend performing a search request in the Time Limit text box.
    To keep from setting a limit, type -1 in this text box.
  5. Enter the time (in seconds) for the server to maintain an idle connection before terminating it in the Idle Timeout text box.
    To keep from setting a limit, type zero (0) in this text box.
  6. Set the maximum number of file descriptors available to the Directory Server in the Max Number of File Descriptors text box. For more information on this parameter, see the Directory Server Configuration, Command, and File Reference.

6.3. Setting Index Scan Limits

In large directories, the search results list can get huge. A directory with a million inetorgperson entries would have a million entries that were returned with a filter like (objectclass=inetorgperson), and an index for the sn attribute would have at least a million entries in it.
Loading a long ID list from the database significantly reduces search performance. The configuration parameter, nsslapd-idlistscanlimit, sets a limit on the number of IDs that are read before a key is considered to match the entire primary index (meaning the search is treated as an unindexed search with a different set of resource limits).
For large indexes, it is actually more efficient to treat any search which matches the index as an unindexed search. The search operation only has to look in one place to process results (the entire directory) rather than searching through an index that is nearly the size of a directory, plus the directory itself.
The default value of the nsslapd-idlistscanlimit attribute is 4000, which is gives good performance for a common range of database sizes and access patterns. It's usually not necessary to change this value. If the database index is slightly larger than the 4000 entries, but still significantly smaller than the overall directory, then raising the scan limit improves searches which would otherwise hit the default limit of 4000.
On the other hand, lowering the limit can significantly speed up searches that would otherwise hit the 4000 entry limit, but where it is not necessary to scan every entry.

6.4. Fine Grained ID List Size

In large databases, some queries can consume a large amount of CPU and RAM resources. To improve the performance, you can set a default ID scan limit that applies to all indexes in the database using the nsslapd-idlistscanlimit attribute. However in some cases it is useful to define a limit for certain indexes, or use no ID list. You can set individual settings for ID list scan limits for different types of search filters using the nsIndexIDListScanLimit attribute.
To set a limit, for example for the objectClass attribute, add the nsIndexIDListScanLimit parameter to the DN cn=objectclass,cn=index,cn=userRoot,cn=ldbm database,cn=plugins,cn=config.
The nsIndexIDListScanLimit attribute is multi valued and takes the following list of parameters as a value: 
nsIndexIDListScanLimit: limit=NNN [type=eq[,sub,...]] [flags=AND[,XXX,...]] [values=val[,val,...]]
  • limit: The maximum size of the ID list. Valid values are:
    • -1: Unlimited.
    • 0: Do not use the index.
    • 1 to the maximum 32-bit integer (2147483647): Maximum number of IDs.
  • type: Optional. The type of the index. eq, sub, pres, and so on. The value must be one of the actual nsIndexType specified for the index definition. For example, you cannot use type=eq if you do not have nsIndexType=eq defined.
  • flags: Optional. Flags that alter the behavior of applying the scan limit. Valid values are:
    • AND: Apply the scan limit only to searches in which the attribute appears in an AND clause.
    • OR: Apply the scan limit only to searches in which the attribute appears in an OR clause.
  • values: Optional. Comma separated list of values which must match the search filter in order for the limit to be applied. Since the matches are done one at a time, the values will match if any of the values matches.
    The values must be used with only one type at a time.
    The values must correspond to the index type, and must correspond to the syntax of the attribute to which the index is applied. For example, if you specified the integer based attribute uidNumber and it is indexed for eq, you cannot use type=eq values=abc.
    If the value contains spaces, commas, NULL, or other values which require to be escaped, the LDAP filter escape syntax should be used: backslash (\) followed by the 2 hex digit code for the character. In the following example, the commas in the DN value are escaped with \2C. 
    nsIndexIDListScanLimit: limit=0 type=eq values=uid=user\2Cou=People\2Cdc=example\2Cdc=com

Example 1. Setting nsIndexIDListScanLimit

In a large database with 10 million entries that contain the object class inetOrgPerson, a search for (&(objectClass=inetOrgPerson)(uid=user)) creates first an ID list containing all 10 million IDs matching objectClass=inetOrgPerson. When the database applies the second part of the filter, it searches the result list for objects matching uid=user. In this cases it is useful to define a limit for certain indexes, or use no ID list at all.
To set that no ID list is created for objectClass=inetOrgPerson in AND clauses, add the following nsIndexIDListScanLimit: 
ldapmodify -D "cn=directory manager" -W -p 389 -h server.example.com -x
dn: cn=objectclass,cn=index,cn=userRoot,cn=ldbm database,cn=plugins,cn=config
changetype: modify
replace: nsIndexIDListScanLimit
nsIndexIDListScanLimit: limit=0 type=eq flags=AND values=inetOrgPerson

modifying entry "cn=objectclass,cn=index,cn=userRoot,cn=ldbm database,cn=plugins,cn=config"
No ID list is created for objectClass=inetOrgPerson when used in an AND clause. In all other situations the value of nsslapd-idlistscanlimit is applied.

6.5. 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, including the server configuration data in the NetscapeRoot database, 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 8.4, “Tuning the Database Cache”, by editing the values for the nsslapd-dbcachesize attribute.

6.6. Tuning the Database Settings for Searches

The Directory Server Console only shows the databases that contain the directory data and the NetscapeRoot database. However, the server uses another database to manage these. On this database, the following attributes can be changed to improve performance:
  • The amount of memory to make available for all databases (maximum cache size), which is described in Section 8.2, “Tuning the Entry Cache”.
  • The maximum number of entries for the server to verify in response to a search request (look-through limit).
  • The amount of memory to make available for import (import cache size).
To configure the default database attributes that apply to all other database instances:
  1. In the Directory Server Console, select the Configuration tab; then, in the navigation tree, expand the Data icon, and highlight the Database Settings node.
  2. Select the LDBM Plug-in Settings tab in the right pane.
    This tab contains the database attributes for all databases stored on this server.
  3. In the Maximum Cache Size field, enter a value corresponding to the amount of memory to make available for all databases. This value is for the total of the entire backend, meaning all databases cumulatively rather than the amount per single database instance.
  4. In the Look-Through Limit field, enter the maximum number of entries for the server to check in response to a search request.
  5. There are two ways to set the amount of memory in bytes to make available for import. The default is to have auto cache sizing, meaning 50% of the free memory is allocated for the import cache. It is also possible to set the import cache size manually by deselecting the Use Cache Auto-Size check box and then setting the value in the Import Cache Size field. For creating a very large database from LDIF, set this attribute as large as possible, depending on the memory available on the machine. The larger this parameter, the faster the database is created.

    Warning

    Setting this value too high can cause import failures because of a lack of memory.

6.7. Managing Special Entries

The cn=config entry in the simple, flat dse.ldif configuration file is not stored in the same highly scalable database as regular entries. As a result, if many entries, particularly entries that are likely to be updated frequently, are stored under cn=config, performance will probably suffer.
Although Red Hat recommends that simple user entries not be stored under cn=config for performance reasons, it can be useful to store special user entries such as the Directory Manager entry or replication manager (supplier bind DN) entry under cn=config since this centralizes configuration information.

7. Tuning Transaction Logging

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 creates a single database transaction for all of the database operations invoked as a result of that LDAP operation. This includes both updating the entry data in the entry index file and updating all of the attribute indexes. If all of the operations succeed, the server commits the transaction, writes the operations to the transaction log, and verifies that the entire transaction is written to disk. If any of these operations fail, the server rolls back the transaction, and all of the operations are discarded. This all-or-nothing approach in the server guarantees that an update operation is atomic. Either the entire operation succeeds permanently and irrevocably, or it fails.
Periodically, the Directory Server (through internal housekeeping threads) flushes the contents of the transaction logs to the actual database index files and checks if the transaction logs require trimming.
If the server experiences a failure, such as a power outage, and shuts down abnormally, the information about recent directory changes is still saved by the transaction log. 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.

Warning

The transaction logging attributes are provided only for system modifications and diagnostics. These settings should be changed only with the guidance of Red Hat Technical Support. Setting these attributes and other configuration attributes inconsistently may cause the directory to be unstable.

7.1. Changing the Location of the Database Transaction Log

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.

Important

If you move the database transaction directory outside the /var/lib/dirsrv directory, then you need to label the new location with the dirsrv_var_lib_t context. Otherwise, SELinux may prevent the server from writing to the directory and database operations may fail.
For example: 
# /usr/sbin/semanage file -a -t dirsrv_var_lib_t -f /new/database-txns/directory
Changing SELinux labels is covered in the Directory Server Administrator's Guide and the Red Hat Enterprise Linux 6 SELinux Guide.
To change the location of the database transaction log:
  1. Stop Directory Server. 
    service dirsrv stop [instance_name]
  2. Edit the dse.ldif file and change the nsslapd-db-logdirectory attribute to show the new log file location. Provide the full path to the log directory in the attribute.
    For information on the nsslapd-db-logdirectory attribute syntax, see the Directory Server Configuration, Command, and File Reference.
  3. Copy the existing transation logs and the DBVERSION file into the new transaction log directory. 
    cp /var/lib/dirsrv/slapd-instance_name/db/{log.*,DBVERSION} /path/to/newdirectory
  4. Restart Directory Server. 
    service dirsrv start [instance_name]

7.2. Changing the Database Checkpoint Interval

At regular intervals, the Directory Server writes operations logged in the transaction log to the database index files and logs a checkpoint entry in the database transaction log. By indicating which changes have already been written to the database indexes, 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 command-line utility to add the nsslapd-db-checkpoint-interval attribute to the cn=config,cn=ldbm database,cn=plugins,cn=config entry. 
ldapmodify -D "cn=directory manager" -W -p 389 -h server.example.com -x

dn: cn=config,cn=ldbm database,cn=plugins,cn=config
changetype: modify
add: nsslapd-db-checkpoint-interval
nsslapd-db-checkpoint-interval: 120
For more information on the syntax of the nsslapd-db-checkpoint-interval attribute, see the Directory Server Configuration, Command, and File Reference.

7.3. Disabling Durable Transactions

Durable transaction logging means that each LDAP update operation, comprised of a sequence of database operations in a transaction, is physically written to disk. Even though each LDAP operation can be comprised of multiple database operations, each LDAP operation is treated as a single database transation. Each LDAP operation is both atomic and durable.

Warning

Turning off durable transactions can improve Directory Server write performance at the risk of data loss.
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:
  1. Use the ldapmodify command-line utility to add the nsslapd-db-durable-transactions attribute to the cn=config,cn=ldbm database,cn=plugins,cn=config entry, and set the value of this attribute to off. 
    ldapmodify -D "cn=directory manager" -W -p 389 -h server.example.com -x

dn: cn=config,cn=ldbm database,cn=plugins,cn=config
changetype: modify
add: nsslapd-db-durable-transactions
nsslapd-db-durable-transactions: off
    For information on the syntax of the nsslapd-db-durable-transactions attribute, see the Directory Server Configuration, Command, and File Reference.
  2. Restart the Directory Server. 
    service dirsrv restart

7.4. Specifying Transaction Batching

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. This is similar to disabling durable transaction logging (in the nsslapd-db-durable-transaction attribute), but setting the batch value gives more control over how many transactions can be potentially lost.
To specify or modify transaction batching while the server is running, use the ldapmodify command-line utility to add the nsslapd-db-transaction-batch-val attribute to the cn=config,cn=ldbm database,cn=plugins,cn=config entry. 
ldapmodify -D "cn=directory manager" -W -p 389 -x

dn: cn=config,cn=ldbm database,cn=plugins,cn=config
changetype: modify
add: nsslapd-db-transaction-batch-val
nsslapd-db-transaction-batch-val: 1
For more information on the syntax and values of the nsslapd-db-transaction-batch-val attribute, see the Directory Server Configuration, Command, and File Reference.

8. Managing the Database Cache Settings

Directory Server uses the following caches:
  • The Entry cache, which contains individual directory entries.
  • The DN cache is used to associate DNs and RDNs with entries.
  • The Database cache, which contains the database index files *.db and *.db4 files.
For the highest performance improvements, configure all cache sizes to be able to store all of their records. If you do not have enough RAM, assign free memory to the caches in the previously shown order.

8.1. Determining the Required Cache Sizes

The dbmon.sh script enables you to monitor cache statistics at runtime and continuously outputs the statistics. To terminate the script, press the Ctrl+C key combination.

Note

The dbmon.sh requires you to pass the options as environment variables to the script. For further details see the Directory Server Configuration, Command, and File Reference.

Example 2. Using the dbmon.sh Script

To display the statistics, enter for example: 
# BINDDN="cn=Directory Manager" BINDPW=secret HOST=server.example.com PORT=389 dbmon.sh
dbcachefree 397310 free% 2.2 roevicts 9282348 hit% 50 pagein 2934772 pageout 219075
      dbname            count          free  free%    size
      userroot:ent      50000          2400    0.8  8972.7
      userroot:dn      100000       4294735   69.8   130.0
To list the total number of DNs in the userroot database, enter: 
# dbscan -f /var/lib/dirsrv/slapd-instance_name/db/userRoot/id2entry.db -t 200 | \
     grep -c rdn:
If your caches are sufficiently sized, the number returned by the previous command matches the value in the count column of the dbmon.sh script's output. Additionally, if all of the entries and DNs fit within their respective caches, the userroot:ent count value matches the userroot:dn count value.
The output of the dbmon.sh script example shows:
  • Only 2.2% free database cache is left: 
    dbcachefree 397310 free% 2.2 roevicts 9282348 hit% 50 pagein 2934772 pageout 219075
    However, to operate efficiently, at least 15% free database cache is required. To determine the optimal size of the database cache, calculate the sizes of all *.db and *.db4 files in the /var/lib/dirsrv/slapd-instance_name/db/ directory including subdirectories and the changelog database, and add 12% for overhead.
    To set the database cache, see Section 8.4, “Tuning the Database Cache”.
  • The DN cache of the userroot database is well-chosen: 
    dbname            count          free  free%    size
userroot:dn      100000       4294735   69.8   130.0
    The DN cache of the database contains 100000 records. 69,8% of the cache is free. Based on the count value and the bytes used, each DN in memory requires 130 bytes on average.
    To set the DN cache, see Section 8.4, “Tuning the Database Cache”.
  • The statistics on the entry cache of the userroot database indicates that the entry cache value should be increased for better performance: 
    dbname            count          free  free%    size
userroot:ent      50000          2400    0.8  8972.7
    The entry cache contains in this database 50000 records and only 2 Kilobytes of free space are left. To enable Directory Server to cache all 100000 DNs, reported by the dbscan utility's output, the cache must be increased to minimum of 856 Megabytes (100000 DNs * 8972,7 bytes average entry size). However, it is recommended to round the minimum required size to the next highest Gigabyte and double the result. In this example, the entry cache should be set to 2 Gigabytes.
    To set the entry cache, see Section 8.2, “Tuning the Entry Cache”.

8.2. Tuning the Entry Cache

The entry cache is used to store directory entries that are used during search and read operations. Setting the entry cache to a size that enables the Directory Server to store all records has the highest performance impact on search operations.
If entry caching is not configured, Directory Server reads the entry from the id2entry.db database file and converts the DNs from the on-disk format to the in-memory format. Entries that are stored in the cache enable the server to skip the disk I/O and conversion steps.

8.2.1. Setting the Entry Cache Size in the Directory Server Console

For example, to set the entry cache for the cn=userRoot database to 2 GB:
  1. Start the Directory Server Console.
  2. Select the Configuration tab and, in the navigation tree, expand the Data icon.
  3. Expand the suffix associated with the database, such as dc=example,dc=com, and then select the database.
  4. In the Database Settings tab, fill the Memory available for cache field and select the unit. For example:
  5. Click Save.
  6. Restart the Directory Server instance: 
    # systemctl restart dirsrv.target

8.2.2. Setting the Entry Cache Size From the Command Line

For example, to set the entry cache for the LDBM database to 2 GB:
  1. Set the value in the Directory Server configuration: 
    # ldapmodify -D "cn=directory manager" -W -x 

dn: cn=database_name,cn=ldbm database,cn=plugins,cn=config
changetype: modify
replace: nsslapd-cachememsize
nsslapd-cachememsize: 2147483648
  2. Restart the Directory Server instance: 
    # systemctl restart dirsrv.target

8.3. Tuning the DN Cache

The entryrdn index is used to associate DNs and RDNs with entries. It enables the server to efficiently perform subtree rename, entry move, and moddn operations. The DN cache is used to cache the in-memory representation of the entryrdn index to avoid expensive file I/O and transformation operations. For best performance, especially with but not limited to entry rename and move operations, set the DN cache to a size that enables Directory Server to cache all DNs in the database.
If a DN is not stored in the cache, Directory Server reads the DN from the entryrdn.db index database file and converts the DNs from the on-disk format to the in-memory format. DNs that are stored in the cache enable the server to skip the disk I/O and conversion steps.
To set the size of the DN cache for the LDBM database to 20 MB:
  1. Update the value in the Directory Server configuration: 
    # ldapmodify -D "cn=directory manager" -W -x 

dn: cn=database_name,cn=ldbm database,cn=plugins,cn=config
changetype: modify
replace: nsslapd-dncachememsize
nsslapd-dncachememsize: 20971520
  2. Restart the Directory Server instance: 
    # systemctl restart dirsrv.target

8.4. Tuning the Database Cache

The database cache contains the Berkeley database index files for the database, meaning all of the *.db and other files used for attribute indexing by the database. This value is passed to the Berkeley DB API function set_cachesize().
This cache size has less of an impact on Directory Server performance than the entry cache size, but if there is available RAM after the entry cache size is set, increase the amount of memory allocated to the database cache.

Note

The operating system also has a file system cache which may compete with the database cache for RAM usage. Refer to the operating system documentation to find information on file system cache settings and monitoring the file system cache.
When setting the database cache value, monitor the cache usage.

8.4.1. Setting the Database Cache Size in the Directory Server Console

For example, to set the database cache to 256 megabytes:
  1. Start the Directory Server Console.
  2. Select the Configuration tab and, in the navigation tree, expand the Data icon.
  3. Select the Database Settings entry.
  4. In the LMDB Plug-in Settings tab, fill the Maximum cache size field and select the unit.
  5. Click Save.
    The Directory Server Console returns an LDAP_UNWILLING_TO_PERFORM error message when you set:
    • a value that is not a number.
    • a value that is too big for a 32-bit signed integer (2147483647) on a 32-bit system.
    • a value that is too big for a 64-bit signed integer (9223372036854775807) on a 64-bit system.
  6. Restart the Directory Server instance: 
    # systemctl restart dirsrv.target

8.4.2. Setting the Database Cache Size From the Command Line

For example, to set the database cache to 256 megabytes:
  1. Update the value in the Directory Server configuration: 
    # ldapmodify -D "cn=directory manager" -W -x

dn: cn=config,cn=ldbm database,cn=plugins,cn=config
changetype: modify
replace: nsslapd-dbcachesize
nsslapd-dbcachesize: 268435456
  2. Restart the Directory Server instance: 
    # systemctl restart dirsrv.target

8.4.3. Storing the Database Cache on a RAM Disk

If your system running the Directory Server instance has enough free RAM, you can optionally store the database cache on a RAM disk for further performance improvements:
  1. Create a directory for the database cache and metadata on the RAM disk: 
    # mkdir -p /dev/shm/slapd-instance_name/
  2. Set the path to the directory on the RAM disk in the nsslapd-db-home-directory attribute: 
    # ldapmodify -D "cn=directory manager" -W -x

dn: cn=config,cn=ldbm database,cn=plugins,cn=config
changetype: modify
replace: nsslapd-db-home-directory
nsslapd-db-home-directory: /dev/shm/slapd-instance_name/
  3. Restart the Directory Server instance: 
    # systemctl restart dirsrv.target

Note

When the database cache is stored on a RAM disk, Directory Server needs to recreate it after each reboot. As a consequence, the service start and initial operations are slower until the cache is recreated.

9. Tuning the Replication Performance

9.1. Improving the Multi-Master Replication Efficiency

The replication latency in a multi-master replication environment, especially if the servers are connected using a wide area network (WAN), can be high in case of multiple masters are receiving updates at the same time. This happens when one master exclusively accesses a replica without releasing it for a long time. In such situations, other masters cannot send updates to this consumer, which increases the replication latency
To release a replica after a fixed amount of time, set the nsds5ReplicaReleaseTimeout parameter on replication masters and hubs. For example, to set a 60 seconds timeout, enter: 
[root@server ~]# ldapmodify -D "cn=Directory Manager" -W -x

dn: cn=replica,cn=dc\3Dexample\2Cdc\3Dcom,cn=mapping tree,cn=config
changetype: modify
replace: nsds5ReplicaReleaseTimeout
nsds5ReplicaReleaseTimeout: 60
The 60 second default value is ideal for most environments. A value set too high or too low can have a negative impact on the replication performance. If the value is set too low, replication servers are constantly reacquiring one another and servers are not able to send many updates. In a high-traffic replication environment, a longer timeout can improve situations where one master exclusively accesses a replica. However, in most cases, a value higher than 120 seconds slows down replication.

11. Improving Import Performance

Very large entry sizes or a large number of entries can negatively impact server performance during import operations. Both Directory Server settings and operationg system settings can be tuned to improve performance for imports.

11.1. Importing Entries with Large Attributes

The nsslapd-cachememsize attribute defines the size allowed for the entry cache.
The import buffer is automatically set to 80% of the cache memory size setting. If the memory cache is 1GB, for example, then the import buffer is 800MB.
When importing a very large database or entries with large attributes (often with values like binary data like certificate chains, CRLs, or images), then set the nsslapd-cachememsize attribute high enough so that the import buffer has enough memory to process the entries.

11.2. Importing Large Numbers of Entries

When there are a large number of entries to be imported, the operating system itself may hit performance limits on what it allows the Directory Server to do. This is particularly true on x86 systems. This can cause import operations to fail because of resource constraints.
If necessary, set the system ulimit value to the maximum number of allows processes for the system user.
For example: 
[root@server ~]# ulimit -u 4096
Then run the import operation.

A. Revision History

Note that revision numbers relate to the edition of this manual, not to version numbers of Red Hat Directory Server.
Revision History
Revision 9.1-9Mon Jun 26, 2017Marc Muehlfeld
Added a statement that this documentation is deprecated and no longer maintained.
Revision 9.1-8Fri Feb 24, 2017Marc Muehlfeld
Added dbmon.sh description. Added "Tuning the Replication Performance" chapter.
Revision 9.1-6Wed Jun 22, 2016Petr Bokoč
Added information to avoid using owner nobody:nobody.
Revision 9.1-5May 23, 2013Ella Deon Lackey
Fixed bugs.
Revision 9.1-1February 21, 2013Ella Deon Lackey
Updates for RHEL 6.4.
Revision 9.0-1December 7, 2011Ella Deon Lackey
Initial version for Directory Server version 9.0.

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