The SAS Disk Spec Gets a Bandwidth Boost

Published: October 6, 2008

by Timothy Prickett Morgan

Now that Serial Attached SCSI (SAS) disks are starting to be deployed as a standard feature on servers of all shapes and sizes--they came to the Power Systems servers this year, after a lot of crabbing--it is time to ramp up the SAS specification and get more bandwidth.

The SCSI Trade Association announced last week that the next-generation SAS-2 disk interface, which sports a 6 Gbit/sec interface, is ready to rock. That's twice the bandwidth available in the current SAS spec, which offers 3 Gbit/sec and the original SAS drives, which came out at a much less useful 1.5 Gbit/sec on the interface coming out of the disk drive.

The updated SAS-2 spec also has wider ports, two ports per drive (instead of one), and full duplex data transmission over the ports. The cables used to link the drives to disk controllers can now be 10 meters, instead of the 6 meter maximum of the 3 Gbit/sec SAS drives. And, of course, SAS-2 disks are going to come out alongside PCI-Express Generation 2 peripheral slots. PCI-Express slots offer data lanes that run at 250 MB/sec and then gang up 2, 4, 8, 16, or 32 lanes to allow peripherals ever-increasing bandwidth back into the system bus. With PCI-Express 2.0, the bandwidth is doubled to 500 MB/sec, which means SAS-2 and PCI-Express 2.0 are kept in relatively stable positions in relation to each other.

In addition to seeing a new range of faster SAS disks in 3.5-inch and 2.5-inch form factors, it will be interesting to see how flash drives are deployed in servers using the faster SAS interface.

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