Tagged command queuing's (TCQ) primary purpose is to enable quorum disk support in OpenVMS clusters.  TCQ is disabled by default because it can interfere with the Tru64 UNIX installation process.  In-house OpenVMS and Tru64 UNIX performance benchmarking indicates that enabling TCQ on all disks provides a small overall improvement in Avanti performance.  Tru64 UNIX versions 4.n and 5.n along with all supported versions of OpenVMS see a benefit.  If performance is critical, test enabling TCQ on all disks.