With the introduction of SAS 12Gbps, seems like "it's time" to do a braindump on SAS.

Work in progress, as usual.

History

By the late '90's, SCSI and PATA were the dominant technologies to attach disks. Both were parallel bus multiple drop topologies and this kind of sucked. SATA and Serial Attached SCSI (SAS) evolved from those, using a serial bus and hub-and-spoke design.

Early SATA/150 and SATA/300 were a bit rough and had some issues, as did SAS 3Gbps. You probably want to avoid older controllers, cabling, expanders, etc. that doesn't support 6Gbps because some of it has "gotchas" in it. In particular a lot of it has 2TB size limitations. Most 3Gbps hard drives are fine though.

Similarities, Differences, Interoperability

SAS and SATA operate at the same link speeds and use similar cabling. SAS normally operates at a higher voltage than SATA and can run over longer cabling.

SAS and SATA use different connectors on the drive. The SATA drive connector has a gap between the signal and power sections, which allows separate power and data cables to be easily connected. The SAS drive connector does not have a gap, and instead has a second set of pins on top. This second set of pins is the second (redundant) SAS port. There are pictures of the top and the bottom of the drive connector.

SATA drives can be attached to a SAS port. Electrically, the SAS port is designed to allow attachment of a SATA drive, and will automatically run at SATA-appropriate voltages. Physically, the SAS backplane connector has an area that will allow either the gapless SAS or the gapped SATA connector to fit. See picture of SAS backplane socket.

SAS drives are incompatible with SATA ports, however, and a SATA connector will not attach to an SAS drive. Don't try. The gap is there to block a SAS drive from being connected to typical SATA cabling, or to a SATA backplane socket.

When a SATA drive is attached to a SAS port, it is operated in a special mode using the Serial ATA Tunneling Protocol (STP).