The IBM 7133 Serial Disk System Advanced Models D40 and T40 provide highly available storage for UNIX and Windows NT servers. By implementing a powerful industry-standard serial technology, the 7133 Advanced Models D40 and T40 provide outstanding performance, availability, and attachability. The rack-mountable 7133 Advanced Model D40 drawer is designed for integration into a supported 19-inch rack. The 7133 Advanced Model T40 is a free- standing deskside tower unit. Both 7133 Advanced Models can be populated with 36.4 GB, 18.2 GB, 9.1 GB, and 4.5 GB IBM Ultrastar disk drives. Drive capacities can be intermixed, providing the flexibility to build storage capacity from gigabytes to terabytes. The 7133 Advanced Models can be intermixed in the same loop with other models of the 7133 (Models 010, 020, 500, 600) as well as with the 7131-405.The 7133 Advanced Models provide outstanding performance for multiple, concurrent, full-duplex I/O operations. Unlike SCSI bus configurations, SSA devices such as the 7133 Serial Disk System are configured in loops and allow multiple concurrent operations to occur in separate sections of these loops. This results in high overall throughput and superior utilization of the 160 MB/sec bandwidth of the 7133 Advanced Models. This SSA capability is referred to as spatial reuse.