ArcGIS 9.0 and later releases allow an ArcSDE geodatabase to be compressed while users are connected.
This functionality is not available in ArcSDE for DB2 until the 9.2 release.
Prior to ArcGIS 9.0, when a database administrator needed to compress an ArcSDE instance, all users had to disconnect. This was required because the compress operation required an exclusive lock on the entire state tree. The exclusive lock prevented any current or future connections until the compress operation completed.
With ArcGIS 9.0 and later releases, users can remain connected during a compress operation. Therefore, users can continue performing spatial operations, editing, and cartographic output. This allows organizations to keep their ArcSDE geodatabase in production during the compress operation, eliminating unnecessary down time.