Commit 3a36d351 authored by Gavin Henry's avatar Gavin Henry
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Merging and amending more FAQ content.

parent f4ca0129
......@@ -4,15 +4,103 @@
H1: Maintenance
System Administration is all about maintenance, so it is only fair that we
discuss how to correctly maintain an OpenLDAP deployment.
H2: Directory Backups
You can use {{slapcat}}(8) to generate an LDIF file for each of your {{slapd}}(8)
back-bdb or back-hdbdatabases.
> slapcat -f slapd.conf -b "dc=example,dc=com"
For back-bdb and back-hdb, this command may be ran while slapd(8) is running.
H2: Berkeley DB Logs
Berkeley DB log files grow, and the administrator has to deal with it. The
procedure is known as log file archival or log file rotation.
Note: The actual log file rotation is handled by the Berkeley DB engine.
Logs of current transactions need to be stored into files so that the database
can be recovered in the event of an application crash. Administrators can change
the size limit of a single log file (by default 10MB), and have old log files
removed automatically, by setting up DB environment (see below). The reason
Berkeley DB never deletes any log files by default is that the administrator
may wish to backup the log files before removal to make database recovery
possible even after a catastrophic failure, such as file system corruption.
Log file names are {{F:log.XXXXXXXXXX}} (X is a digit). By default the log files
are located in the BDB backend directory. The {{F:db_archive}} tool knows what
log files are used in current transactions, and what are not. Administrators can
move unused log files to a backup media, and delete them. To have them removed
automatically, place set_flags {{DB_LOG_AUTOREMOVE}} directive in {{F:DB_CONFIG}}.
Note: If the log files are removed automatically, recovery after a catastrophic
failure is likely to be impossible.
The files with names {{F:__db.001}}, {{F:__db.002}}, etc are just shared memory
regions (or whatever). These ARE NOT 'logs', they must be left alone. Don't be
afraid of them, they do not grow like logs do.
To understand the {{F:db_archive}} interface, the reader should refer to
chapter 9 of the Berkeley DB guide. In particular, the following chapters are
- Database and log file archival
- Log file removal
- Recovery procedures
- Hot failover
Advanced installations can use special environment settings to fine-tune some
Berkeley DB options (change the log file limit, etc). This can be done by using
the {{F:DB_CONFIG}} file. This magic file can be created in BDB backend directory
set up by {{slapd.conf}}(5). More information on this file can be found in File
naming chapter. Specific directives can be found in C Interface, look for
{{DB_ENV->set_XXXX}} calls.
Note: options set in {{F:DB_CONFIG}} file override options set by OpenLDAP.
Use them with extreme caution. Do not use them unless You know what You are doing.
The advantages of {{F:DB_CONFIG}} usage can be the following:
- to keep data files and log files on different mediums (i.e. disks) to improve
performance and/or reliability;
- to fine-tune some specific options (such as shared memory region sizes);
- to set the log file limit (please read Log file limits before doing this).
To figure out the best-practice BDB backup scenario, the reader is highly
recommended to read the whole Chapter 9: Berkeley DB Transactional Data Store Applications.
This chapter is a set of small pages with examples in C language. Non-prorgamming
people can skip this examples without loss of knowledge.
H2: Checkpointing
If you put "checkpoint 1024 5" in slapd.conf (to checkpoint after 1024kb or 5 minutes,
for example), this does not checkpoint every 5 minutes as you may think.
The explanation from Howard is:
'In OpenLDAP 2.1 and 2.2 the checkpoint directive acts as follows - *when there
is a write operation*, and more than <check> minutes have occurred since the
last checkpoint, perform the checkpoint. If more than <check> minutes pass after
a write without any other write operations occurring, no checkpoint is performed,
so it's possible to lose the last write that occurred.''
In other words, a write operation occurring less than "check" minutes after the
last checkpoint will not be checkpointed until the next write occurs after "check"
minutes have passed since the checkpoint.
This has been modified in 2.3 to indeed checkpoint every so often; in the meantime
a workaround is to invoke "db_checkpoint" from a cron script every so often, say 5 minutes.
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