When should I manually defragment a domain controller?

An online defragmentation of the Active Directory (AD) database occurs during the (AD) Garbage Collection process that runs every 12 hours by default. This defragmentation optimizes the database’s structure but doesn’t shrink the database’s size. If you’ve deleted a large number of records from AD or, more likely, you’ve disabled a DC as a Global Catalog (GC) server and you have multiple domains, AD might benefit from an offline defragmentation to shrink the physical size of the AD database.