Compaq ML350 Serial Attached SCSI storage technology, 2nd Edition - Page 7

Figure 3. SAS 1.1 at 1.5 Gbps and 3.0 Gbps in untrained mode has a 6-m limit. SAS-2 in trained mode raises the limit to 10 m at all speeds. At 6 Gbps, active copper and optical cables extend the distance to 20 m. Storage power management SAS-2.1 devices will be able to turn off SAS physical links when they are idle. Each SAS transceiver consumes about 200 mW. Therefore, the total power savings from a dual-ported drive with two transceivers and a controller (or attached SAS expander) with two transceivers is a little less than 1 W. Table 3 includes an example of power savings for a 9-W small form factor (SFF) SAS drive. Future SAS and SATA disk drives will include more standardized power management features. Today's drives support the Active and Stopped states. The Stopped state is rarely used due to its long recovery time. Table 3. Power management state example for a 9-W SFF SAS drive State Active Idle Description Fully active Stop clocks to idle circuitry Commands processed Yes Yes Power savings Recovery time None None ~1 W < 1 ms Park heads Retract the heads from the media, reducing drag Automatic state change ~2 W 1 sec Low-rpm standby Standby Stopped Retract heads, spin down to a lower rpm Automatic state change Retract heads, spin down to 0 rpm, cache still powered Automatic state change Same as Standby No ~4 W ~7 W ~7 W 7 sec 15-20 sec 15-20 sec Sleep Everything off except No wakeup circuitry ~9 W 30 sec 7