Whys and Wherefores of SAS

Version 1.0 of the SAS spec was just completed by the SCSI Trade Association.

This article can also be found in the Premium Editorial Download: Storage magazine: Should you consolidate your direct-attached storage (DAS)?:
For more than 20 years, disk drives and systems have connected across a parallel bus interface, which is now giving way to a serial architecture--Serial ATA (SATA) drives are case in point. And then there are Serial-Attached SCSI (SAS) drives. Version 1.0 of the SAS spec was just completed by the SCSI Trade Association. SAS, due out in less than a year's time, should present a host of improvements over parallel SCSI, including:
  • Thinner cables. Serial cables are only 5/16," compared to a 1 3/4" parallel SCSI ribbon, for better air flow.
  • Better connectivity. SAS calls for up to 4,000 devices per port, and 128 drives per channel. Practically speaking, it will be possible to connect hundreds of SAS drives together.
  • Improved performance. First generation SAS drives will have a 3Gb connection, about the same as Ultra320 drives. But because SAS drives are connected point-to-point, there's no bandwidth contention across a shared bus.
  • Dual connectors. Data travels full-duplex at 3Gb/s.
  • Compatibility with SATA drives. When behind a SAS controller chip, a single disk enclosure will be able to contain both SAS and SATA drives.
This was first published in June 2003

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