Sas-2; Second Generation Serial Ata (Sata); Interconnect Bandwidths And Drive Throughput - Compaq BL10e - HP ProLiant - 512 MB RAM Technology Overview

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
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