Double Standard: SAS and SATA

Customers request servers or storage arrays with drives based on the serial-attached SCSI (SAS) standard for a variety of reasons. The fact that SAS is the latest and most sophisticated in performance is one. Its ability to cascade thousands of drives to one interface is another.

Yet a third key motivation of moving to SAS is the ability to build tiered storage solutions on a common backplane consisting of both SAS and the less expensive but high-capacity serial ATA (SATA) drives. Such a solution offers what promises to be a cost-effective and easy-to-manage single platform for running both mission-critical online storage with nearline storage.

Historically, the two types of drives have run on separate platforms because they're based on different interfaces. But disk-drive vendors and suppliers of related components, such as controllers and host-bus adapters, remedied that in the development of the SAS spec by allowing SATA drives to connect to the SAS architecture. (Conversely, it should be noted that an SAS drive can't run on a SATA-only platform.) Among those that very early on promoted the ability to mix and match SATA and SAS drives was Maxtor, which staged a public demonstration of a prototype in the summer of 2004--more than a year before the first SAS drives appeared in the channel.

Michael McDonald, senior director of marketing for storage products at Broadcom, says he is seeing interest in high-performance mirroring solutions in which online data is stored on SAS drives and lower-performing targets are SATA-based.

"Instead of having a RAID mirror where everything is using the high-performance drives, you have half the mirror using SATA," McDonald says. "That is gaining a lot of interest."

The ability to mix and match also allows solution providers to build high-performance RAIDs using SAS drives, while having SATA drives as spares.

"Instead of having your hot standby be very expensive drives, why not just have low-cost drives?" McDonald says.

The issue, however, is that when a RAID array fails and standby SATA drives are brought in, the overall performance of the RAID solution goes down.

"That's something the jury is still out on--how well that model will be accepted," McDonald says.

The Challenges

Other issues exist as well. While Maxtor affirms that the two drive types can run on a common backplane, others warn that this capability may not be ready for prime time.

"At this point, you don't want to plug SATA and SAS drives on the same backplane inside the same box," warns Don Jeanette, senior product marketing manager at Fujitsu Computer Products.

The reason, Jeanette and others say, is that 99 percent of the time, SATA drives will be running at different speeds than SAS drives; the former runs at 4,200, 5,400 and 7,200 rpm, while SAS drives run at 10,000 and 15,000 rpm.

As a result, mechanical vibration and rotational issues could affect the performance of the drives if they are too close together.

"I hope every vendor out there would admit a lot of fleshing out and testing needs to be done," Jeanette says.

Charlie Kraus, director of host-bus adapter products at LSI Logic, agrees, saying he has specifically advised VARs not to bolt SATA and SAS drives on the same backplane.

"We've killed SATA drives in a matter of weeks doing this because of the different rotational speeds," Kraus notes. "SATA drives are not at all designed to tolerate this stuff."

That doesn't preclude installing the two in the same chassis, he explains, so long as they're not on the same backplane. Within the chassis, an integrator could have two backplanes, one for SAS and one for SATA, as long as they are mechanically isolated to avoid mechanical cross-talk, he adds.

In instances where the drives need to be on the same chassis, Kraus advises adding rubber mounting to avoid shock and vibration across the two drive types.

The Workarounds

Maxtor, clearly a champion of mixing and matching SAS and SATA drives, says while such concerns are valid, they can be overcome. The company's recently released MaxLine 500, for example, supports what the company calls "rotational vibration compensation." As the term implies, the drives can withstand different vibrational and mechanical speeds and compensate for them accordingly, Maxtor says.

"It works particularly well in mixed environments without any issues," says Marty Czekalski, Maxtor's interface architecture initiatives manager, and vice president of the SCSI Trade Association, a consortium of vendors that holds interoperability tests, called Plugfests, several times a year.

Indeed, if implemented correctly, the two drive types should be able to cohabitate on the same backplane, asserts Jason Blosil, senior manager of channel marketing at Adaptec. Blosil says many VARs are already building JBOD solutions that do so with the company's new SANbloc S50 JBOD, a 2U-based solution.

"It depends on the chassis design," Blosil says. Based on the 3-x-4 drive configuration of the chassis, for example, Adaptec recommends putting SAS and SATA drives in separate columns to avoid the mechanical issues.

It's too early in the life cycle of SAS products, which vendors only started shipping over the past several months, to get a handle on how they will be used in these pooled configurations. Certainly for larger-scale systems, the need to run SAS and SATA drives on a common backplane may be moot as solution providers and OEMs would ultimately deploy them on dual or multiple backplanes, keeping the drive types segregated. But observers say their ability to share the same backplane is equally significant for other reasons--notably, to mix and match components for the upgrading or downgrading of storage within the same server or storage array.

"The prospect of having a common infrastructure that supports both SAS and SATA drives is tremendously valuable," says Geoffrey Noer, senior director of product management at Rackable, an Adaptec systems-builder partner.