DEC DSSI VMScluster Installation and Troubleshooting Guide (EK-410AB-MG. D01) talks about all the different ways to cluster AXPs and VAXen using DSSI (but doesn't cover PCI, only EISA + proprietary DSSA adaptors). Evidently, DSSI clusters were used often enough. Seems that by this date, they were implemented mostly with HSDs rather than DSSI drives.