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Latest SATA spec addresses density issues

SATA-IO has quelled some, but not all, previous problems with the latest revision of the SATA specification.

The Serial ATA International Organization (SATA-IO) announced that they have consolidated previously scattered work on SATA technology with a new spec, SATA Revision 2.5, that will be available by mid-November.

The main difference between Revision 2.5 and previous specs is that seven previously separate documents, most centered around cabling and connectivity between SATA drives, have been pulled into Revision 2.5. The documents include: SATA 1.08, the original SATA spec; the SATA extension, which defines many of the core next-generation SATA features, such as higher throughput and native command queuing (NCQ); two separate cabling and connectivity specs known as Volume 1 and Volume 2; a port multiplier spec; a port selector spec; and lastly, a spec for the 3 Gbps throughput rate.

According to analysts, the new spec addresses some previous issues with storage density and connectivity. However, difficulties defining and naming next-generation SATA products remain.

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When last we left SATA II (" SATA II still on rocky road", June 26), concerns had been discussed during a convention put on by Western Digital Corp. and LSI Logic Corp. about a "perfect storm" of problems associated with higher-performing, denser low-end drives, most of them having to do with the logistics of having more drives in a tighter space.

While serial drives use smaller wires than the ribbon cables required for parallel ATA systems, packing more drives together can make even skinnier serial connections obtrusive.

The new spec addresses that concern, according to Greg Schultz, senior analyst with the Evaluator Group, by pulling in port multipliers.

"First generation SATA was limited to a master/slave relationship between disks," he said. "With this new spec, you have multiple drives daisy-chained together using port multipliers, which allow you to have more drives attached to a controller or adapter with less wiring than in the past."

Other connectivity specs included in Revision 2.5 allow for "multi-lane" cables aimed at enterprise data centers where storage density could become overwhelming, according to Knut Grimsrud, SATA-IO president and chairman.

The four-lane cable, meant to connect the server to the port multipliers in the storage box, "is another ingredient that allows denser packaging at the enterprise level," said Grimsrud.

Then again, this is all theoretical -- port multipliers and multi-lane cables will only help if vendors decide to include them in their products. And under the SATA specs, including Revision 2.5, most of these 'ingredients' still aren't required.

To be identified as next-generation SATA, a product must offer one of the two main features, 3 Gbps throughput or NCQ. Though as of Revision 2.5 the SATA spec stands at 600 pages, the rest is optional.

"To me, it almost sounds like a compromise by committee on what they will or will not call (next-gen SATA), depending on what some vendors have or will have, so that others don't get left out," Schultz said. "If one vendor comes out and has the 3 G[bps] and NCQ but not all of the other features, they still want to be considered part of the new umbrella."

What's in a name?

Moreover, even what to call products that meet the spec has yet to be addressed completely by SATA-IO.

The group has clarified at least part of the mystery -- next-generation SATA products should not be called SATA II, as they have been in the past. What the group still won't say is what they should be called instead.

Instead, SATA-IO released a lengthy document on its Web site detailing the many possibilities of naming conventions for next-generation SATA products ("Dispelling the Confusion: SATA II does not mean 3Gb/s"), but demurred from picking any one of them.

"They've got some explaining to do," Schultz said. "If they don't want to call it SATA II, what do they want to call it? They need to either tell people explicitly what it is, or live with the brand recognition that they have."

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