When to offline defrag the Active Directory database

By | June 12, 2011

This article will show a simple way to determine if there is any gain to do an offline defrag of your Active Directory database.

During normal operations the Active Directory service will do an online defragmentation of the Active Directory database (always called ntds.dit) each 12 hours. This online defrag will arrange all pages in an optimal way internal in the ntds.dit, however the file size will never shrink, sometimes even grow. During the years of operations of the ntds.dit the file size will increase as user accounts, organizational units, groups, computers, dns records and more are added and later removed. When deleted objects are finally removed (after the so called tombstone lifetime, typically 180 days) the space they have occupied will unfortunately not decrease.

The actual size of the ntds.dit could be easily studied through Explorer, as above. The size of the database is in this example around 575 MB. Note that Active Directory does not use a file level replication, so the file could be of various size on each Domain Controller in your domain. If wanted there is the possibility to take the AD services offline on one DC and then do an offline defragmentation of ntds.dit. This would both arrange all pages the best possible way, and also to reclaim any empty space inside the database, which could make backup and restore faster and also possible increase AD performance.

The offline defrag means “offline” from an Active Directory perspective. This means that on Windows 2000 and 2003 you will have to reboot into Directory Services Restore Mode, and on Windows 2008 and R2 you will have to stop the AD services by typing “net stop ntds” in the command prompt. So in Windows 2008 and later it is far easier, but still something that you do not want to do if not necessary.

There are numerous article on the web how to do the actual offline defrag, so we will not cover that part here. However, we will see the perhaps most important information and that is to be able to see in advance the amount of space that we could reclaim. With this information we could make our decision based on fact and not guesses. This has been possible since at least Windows 2003, but is not well documented.

To enable this you will have to alter a registry value on the Domain Controller you will investigate the reclaimable MBs. Use regedit and find the following key:

HKEY_LOCAL_MACHINE \ System \ CurrentControlSet \ Services \ NTDS \ Diagnostics

Change the value “6 Garbage Collection” from 0 to 1. This will increase the logging from the Garbage Collection process which runs together with the online defrag. So now wait for the next online defragmentation which runs twice a day and then study the Directory Service log in Event Viewer.

Search for event id 1646, usually together with event ids 700 and 701.

Here we can note the amount of space that would be reclaimed from an offline defrag. The top value is the number of MB that the offline defrag would recover, here almost half the database size. If the amount is negligible then do not worry about this any more, and if there is a considerable amount of MBs reported then you could plan to do the offline defrag.

Note that both the change of registry key and the actual offline defrag has to be done on each domain controller, since neither does replicate.

As noted above we will not look at the commands for the offline defragmentation here, since they are well documented already. Read for example this Microsoft KB article for a guide to the steps involved.

3 thoughts on “When to offline defrag the Active Directory database

  1. henrik hallebrand

    The registry path is wrong, it should be Diagnostics at the end and not Parameters 🙂

  2. Rickard Nobel Post author

    Thanks Henrik! The picture was correct, but mis-typed in the text. The path has now been updated.

  3. Pingback: How to compat AD database (offline defragmentation)? | Jacques DALBERA's IT world

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