In Office 365, many administrators use Directory Synchronization, providing a centralized location to manage all users, contacts and groups (local Active Directory) and have those updates synchronized into Office 365. Once Directory Synchronization has been enabled in Office 365, the DirSync application can start synchronizing Active Directory objects. If at any point Directory Synchronization is disabled within the Office 365 service, via the Microsoft Online Portal (MOP), and then later enabled, the Directory Synchronization appliance will be instructed to perform a FULL SYNCHRONIZATION.
This is different from BPOS DirSync and administrators in large Active Directory environments show keep this in mind, as a Full Synchronization will wipe the DirSync database and start over. This can take significant time to complete and should be taken into account when discussing whether to temporarily disable Directory Synchronization for your Office 365 online tenant/company.
Note – The above describes a scenario where you have been Directory Synchronizing, where Directory Synchronization is running in a Delta state, which means only changes to Active Directory object attributes and values are synchronized. Disabling DirSync, even temporarily, and then enabling this service will force DirSync to perform a Full Sync, taking administrators by surprise and may find that updates for changed AD users, contacts and/or groups are not being seen as updated as quickly as before. This would be due to a full Directory Synchronization taking place.