Synergetic SANs and blades

Synergetic SANs and blades

"Synergetic (adj): Working together; used especially of groups, as subsidiaries of a corporation, cooperating for an enhanced effect."

I like this short definition of the word synergetic because it's a good description of what happens when you join blade servers and SANs.

Obviously, dedicating storage devices to each server doesn't fit well with the cooperative nature of blade clusters. That's why they usually connect to their storage via some kind of switching device, such as an FC (Fibre Channel) switch.

However, that may change, because SAS (serial attached SCSI) -- an emerging technology that will eventually replace parallel SCSI in enterprise storage -- is acquiring new capabilities. (For more on SAS, check out this Storage Network blog post from March 17.)

It's difficult to argue that SAS isn't an ideal partner for server blades, considering its well-aligned characteristics, including high scalability, small-form-factor drives, and controller compatibility with less-expensive SATA (serial ATA) drives.

But there's more: Unlike its parallel ancestor, the SAS protocol makes it possible to connect thousands of devices -- at least it does on paper. Practical real-world applications may deploy much smaller numbers, say hundreds of devices.

Now imagine an installation with blade servers connecting their storage enclosures, each mounting a dozen or so SAS drives. There's plenty of capacity there, but to attain proper switching functionality, we would have to insert Ethernet or FC switches in that configuration.

See anything wrong with this picture? Using the FC or Ethernet switches makes the setting complicated and expensive. PMC-Sierra agrees, and in cooperation with major blade vendors such as Dell, HP, and IBM, it is doing something to improve the situation by adding switching capability to its SAS adapters.

Quite a few boxes in your datacenter already may have been built using PMC-Sierra components or designs, so this move isn't a huge leap for the company. If you want to know more, PMC-Sierra's Web site is worth visiting, if for no other reason than to access Webinars and white papers on a variety of interesting topics, including SAS. Registration is needed.

"We recognized an opportunity to take the SAS standard and add the switching features that you would normally see in FC or Ethernet," said Mark Stibitz, vice president and general manager of the enterprise storage division at PMC-Sierra.

In fact, PMC is announcing two new products in its maxSAS line. One offers a 24-port connection to SAS enclosures and the other offers a 36-port connection. These cards will allow switched connections between blade servers and SAS enclosures. Moreover, like FC switches, they will make it possible to restrict blade servers' access to storage via zoning and access control.

These new zoning and access control features are quite a departure from the original SAS specs, which required building an extension to the standard and pushing it through the T10 committee, Stibitz explains.

"This will enable servers' OEM to add zoning and security to a SAN fabric and allows them to build a new class of blade servers," he says, adding that SAS-based routing should reduce the cost of blade-server solutions and improve performance by avoiding unnecessary protocol conversions.

In addition to the two new cards -- priced at US$59 and US$89 each for 10,000 units -- PMC is also offering a US$4,000 Reference Kit that includes a card, switch-management software, and cables. Basically, this kit gives you everything you'll need to build and test an SAS-switching prototype.

SAS switching is certainly an interesting new twist for storage and for blade servers, but don't count out other technologies just yet. In a quasi-simultaneous announcement, Emulex and HP introduced new 2Gb FC cards, based on Emulex technology, for the HP BladeSystem.

I am not surprised: FC switches and HBAs are the foundations of our SANs and will not be replaced anytime soon. Besides, SAS has a 3-meter limitation to which FC is immune.

Regardless, it's always good to see new technologies challenging the incumbent. It certainly makes life more interesting and keeps costs under control.

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