Article

Dance of the disk drives, Page Three

Michael Desmond
Page Three
Serial switch over

Drive makers are replacing a pair of venerable storage interfaces--ATA and SCSI--with new serial-based offerings. Serial ATA (SATA) and Serial-Attached SCSI (SAS) offer extended features, improved management and a fresh performance roadmap. It's a transition with far-reaching implications because everything from disk arrays to server backplanes must adapt to the new interfaces.

SATA drives and compatible systems have been shipping for more than six months and are rapidly displacing parallel ATA solutions. Some SATA-savvy array products include EMC Corp.'s Clariion and Network Appliance Inc.'s NearStore R200. LSI Logic Corp. also produces a pair of SATA-capable arrays for its OEM customers. With 1.5Gb/s data rates, SATA boasts a well-formed roadmap that includes provisions for command queuing and reordering, multiple ports and a one-meter cable for enhanced design flexibility. SATA is also being built into new array offerings, which could push established vendors to roll out SATA-enabled midrange arrays of their own.

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Article at a glance
1-Tracking disk technology
2-Flattening capacity curve
3-Serial switch over
4-Form over function
5-Category crossovers

Steve Kenniston, technology analyst for the Enterprise Storage Group (ESG), says demand for affordable, high-capacity disk storage is driving SATA. A recent survey by ESG indicates that nearly 25% of respondents currently use disks in their backup environment. "That's climbing to 80% in the next two years," says Kenniston.

Future versions of SATA will boost data rates to 3Gb/s, while remaining backward compatible with 1.5Gb/s SATA gear. A 6Gb/s SATA spec is on the roadmap, though drives based on this technology are years away.

The recently approved SAS standard will take longer to arrive. Prototype products--from Maxtor, Seagate and others--have appeared at trade shows, and the SCSI Technical Association (STA) has been conducting plug-fests to ensure interoperability. The new standard offers 4Gb/s data rates out of the box, while the SAS roadmap calls for transfer rates as high as 12Gb/s.

Like SATA, SAS improves flexibility, providing support for hardware that can theoretically support more than 16,000 drives. In an interesting twist, the SAS interface is actually a superset of the SATA pinout, which means a SAS backplane can support both SAS and SATA drives. And like SATA, the SAS interconnect employs more compact cabling and connectors, which simplifies management and streamlines server and chassis designs. The first SAS products should hit the market at the end of 2004 or early 2005.

"You still have a pretty significant legacy volume of Ultra 320 SCSI through 2005," says Joel Hagberg, vice president of marketing at Fujitsu. "As you go into '06, most manufacturers are into the end-of-life planning for U320 SCSI."

Stephanie Balaouras, senior analyst in the enterprise computing and networking practice at the Yankee Group, expects SAS products to be limited to direct-attached storage (DAS). She cites the falling cost of Fibre Channel drives and the increasing reliability and utility of SATA drives, creating a squeeze in the middle for late-arriving SAS products.

Kenniston is more direct: "I only know of one company pushing Serial-Attached SCSI, and that's HP. It's on all the vendor roadmaps to have something, but I don't see SAS being their number one priority," Kenniston says.

International Data Corp. expects SAS hard drive production to reach 8.1 million units by 2007. By comparison, some 8.8 million parallel SCSI drives and 8.5 million Fibre Channel drives are expected to ship in the same timeframe. One thing that will delay volume sales of SAS drives is the transition of server backplanes from parallel SCSI to the serial variant.

Recommendation: IT shops should deploy servers that provide native support for SATA and/or SAS as they become available, in order to prepare for future drive deployments. Start planning to incorporate SATA arrays into your storage environment for nearline disk storage.

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