Compaq ProLiant 1000 Drive technology overview - Page 8

HP was instrumental in developing the SAS standard. For a more detailed discussion of SAS, see the HP technology brief entitled "Serial Attached SCSI technology" available at http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01613420/c01613420.pdf. SAS-2 The SAS-2 interface supports a link speed of 6 Gb/s, or 600 MB/s, in each direction. SAS-2 controllers were first introduced with HP ProLiant G6 servers in 2009. The SAS-2 interface will support SAS and SATA drives; however, the 6 Gb/s link speed is only achieved using new SAS-2 compliant disk drives. SAS-2 also supports SAS expanders to create a SAS fabric supporting large numbers of drives on a disk controller. This concept was introduced with SAS and has continued to evolve. SAS-2 can theoretically support an unlimited number of drives in a SAS fabric, although the practical limit is typically 512 - 2048 drives. This is based on the maximum size of the routing tables supported in the SAS expanders. Second generation Serial ATA (SATA) SATA is usually considered the best solution for price-sensitive, low I/O workload server applications, such as entry level and external storage. SATA is also expected to dominate the desktop market due to its low cost. SATA introduces a serial communication interface that operates in simplex mode, increases data transfer rate, requires a small-diameter cable, supports additional disks, and supports hot swapping. SATA uses a seven-pin, small-diameter cable. The controller may be built into the mother board or provided as an adaptor card. The SATA specification was initially released in three variants:  1.5 Gb/s  1.5 Gb/s with extensions  3.0 Gb/s The initial SATA 1.5 Gb/s variant was targeted at replacing ATA in the desktop and consumer markets. It introduced a serial interface that supports one drive per controller port. SATA 1.5 Gb/s with extensions is targeted to workstations and low-end servers. It adds native command queuing. SATA 3.0 Gb/s is targeted to workstations and low-end servers. It increases the data transfer rate. For a more detailed discussion of SATA, see the technology brief entitled "Serial ATA technology" at http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00301688/c00301688.pdf Interconnect bandwidths and drive throughput Both SATA and SAS channel bandwidths are now double their previous rates, allowing twice as much data to be delivered across the interfaces. Drive throughputs, the maximum rates at which data can be read from a drive, have also been increasing. It is important to understand the interaction of these two factors, especially when overall performance is a consideration. The new SAS-2 interface operates at 6 Gb/s, giving it an effective maximum theoretical bandwidth of 600 MB/s. Current SAS disk drives are capable of a maximum sustained throughput of about 175 MB/s, or less than one-third the capacity of the SAS-2 channel. The implications of this are fairly straightforward. The 6 Gb/s SAS-2 interface and SAS-2 compliant drives provides little or no performance advantage over standard 3 Gb/s SAS when used in configurations where single drives 8