Serial Attached SCSI Storage for High-Performance Computing

Serial protocols are becoming widespread as performance requirements advance beyond what traditional bus-based systems can provide. Traditionally, SCSI disks are connected to a shared parallel bus. However, the accuracy of parallel connections decreases at high speeds and performance requirements limit the bus length.

The Serial Attached SCSI (SAS) protocol is designed to help avoid these shortcomings. Serial connections between devices can provide much higher throughput than parallel bus-based connection schemes. The current SAS technology uses point-to-point connections with a maximum 300 MB/sec capacity per disk — in contrast, SCSI disks share a bus that has a maximum 320 MB/sec capacity. The SCSI Trade Association expects the performance of SAS links to double with SAS 600 (expected to be available in 2007) and double again with SAS 1200 (expected to be available in 2010).  Additionally, SAS overcomes the SCSI limit of 16 devices per channel. As a result of these improvements, even though SAS uses the same set of SCSI commands, it is not backward compatible with SCSI.