When you delete an object from AD, it gets tombstoned i.e. not deleted but stored in tombstone for a period of time in case you want to restore it back (180 days in win 2008 by default). Once 180 days completes that object is considered to be of no use any more and can be cleaned from the database and free up some space. The cleanup process is done by Garbage Collection. Garbage collection in Active Directory Domain Services (AD DS) is the process of removing deleted objects (tombstones) from the directory database. This process results in free disk space in the directory database.
By default, this free space is not reported in Event Viewer. To see the amount of free disk space that can be made available to the file system by offline defragmentation, you can change the garbage collection logging level so that the disk space is reported in the Directory Service event log. After you change the logging level, check the Directory Service event log for Event ID 1646, which reports the amount of disk space that you can recover by performing offline defragmentation.
The garbage collection logging level is an NTDS diagnostics setting in the registry. You can use this procedure to change the garbage collection logging level to 1 so that you can view Event ID 1646 in Event Viewer.
How to change the garbage collection logging level
- Click Start, click Run, type regedit, and then press ENTER.
- In Registry Editor, navigate to the Garbage Collection entry inHKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NTDS\Diagnostics.
Double-click Garbage Collection. In the Value data box, type 1, and then click OK.
Now you must wait for the on-line defrag to occur on the NTDS.dit database. Then Event 1646 shows up in the Directory Service Log.