I was curious to see how long online initialization would take with our server setup. First of all i wanted to test how much time slapadd takes with our user database. Keep in mind that we keep quite a lot of attributes in our user entries (the inetorg* set of objectclasses, eduperson and various service specific attributes), so we ‘ve ended up configuring 20 indexes on our database. That probably means that slapadd will have a lot of work to do when adding entries.
Plain slapadd took around 10 minutes to complete for 23,000 entries while slapadd with the -q option takes only 1,5 mintues!! Talk about a speedup. So it seems the -q option is a must.
The main difference between slapadd and online initialization though is that the later will go through all the modules configured on the ldap server. In our case this configuration includes:

  • Changelog overlay (only added to the server configuration for the last init run).
  • Audit log (thus all operations will get logged on the audit log file).
  • constraint and attribute uniqueness overlays.
  • Schema checking.

Consequently, I would assume that online initialization would take more time than the original slapadd (since it actually has to update two databases, the actual user database and the accesslog db as well as log the operations to the audit log).

(note:  Please don’t take the actual init times into account, just the difference between them. Right now both replicas are hosted on the same machine as jails so i ‘m planning to update the numbers when i move one to another machine).

The first try on online init failed and seemed to get stack somewhere. Adding Sync to the value of olcLogLevel under cn=config can be very helpful on troubleshooting. It turned out that there’s an obvious chicken-egg problem with the value list attribute constraint of the constraint overlay. When setting up a value list attribute constraint, the list is taken from the result of an LDAP URL. Since during initialization there are no entries in the directory, that LDAP URL will return an empty list and as soon as you try to add an entry with an attribute corresponding to a value list constraint the operation will fail and initialization will get halted. The solution was to disable the value list constraint until initialization finished.

Next try was a success but took much more time than anticipated.  One tuning action that had nice results was to increase the checkpoint interval. At first i had configured checkpoint to happen every 60KB’s on the user database and 120KB on the changelog database. Since i also had configured checkpoints to happen every 5 minutes it was a good idea to increase the size to 1MB. This  dropped online initialization to 22 minutes (no accesslog overlay), more than double the time for plain slapadd to run on the same data.

Just to have an idea on the performance increase of checkpoint tuning here are the results on my test platform (consumer has a minimum configuration with just the back-hdb configuration and no changelog, auditlog, unqiueness and constraint overlays):

  • Checkpoint Size: 120KB, Init Time: 14 minutes
  • Checkpoint Size: 1MB, Init Time: 7 minutes (50% decrease!)

After enabling the accesslog overlay, the increase in total operation processing was quite impressive since this time online init took around 33 minutes (compared to 22 minutes).

UPDATE:

Since syncrepl will fall back to full present/delete phase if the changelog does not contain enough information or is outdated, it becomes clear that the accesslog database and it’s contents are not mission critical. Consequently, it is advisable to enable the dbnosync configuration directive for the accesslog database. Enabling the setting dropped online init time to around 25 minutes, almost removing the additional accesslog update cost.

Lastly, a couple more notes on tuning the BDB database:

  • db_stat -m will give you statistics on the BDB cache usage and hit ratio. It can be a helpful tool to check out if your BDB cachesize setting (set_cachesize in DB_CONFIG) is working properly or is set too low.
  • This wiki page on Sun (although targeted on Sun DS) can provide useful hints on determining if your database page size is optimal for your configuration. In OpenLDAP back-hdb the corresponding configuration directive is dbpagesize.
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