Suresh Paniker, director of worldwide marketing at Adaptec, estimates that 20% or fewer of Adaptec’s
SATA customers are taking advantage of RAID 6 today, but that use of RAID 6 will pick up because the
associated write penalty is no longer an issue.
Mix n’ match SAS, SATA
RAID 6 may be important for any disk array populated with SATA drives, but it becomes even more important
as end users and integrators increasingly put SATA drives in SAS enclosures-sometimes mixing the two drive
types-and deploy them in mission-critical applications where drive/array reliability is critical.
The ability to put relatively expensive, high-performance SAS drives in the same enclosure as low-cost, high-
capacity SATA drives is often touted as the key advantage of SAS. However, just because you
can
intermix
drive types doesn’t mean you
should
.
Users and systems/storage integrators experienced a number of problems when they were trying to intermix the
two drives types in early implementations of SAS subsystems. Some of those problems were related to
vibrations from high-speed 15,000rpm SAS drives causing errors on, or failures of, 7,200rpm SATA drives, as
well as performance degradation on the SATA drives.
Controller and subsystem vendors are addressing those issues with recommendations regarding how users and
integrators should arrange the drives (for example, putting drive types in their own vertical columns). Another
recommendation from some vendors is to put each drive type behind a different controller.
“We recommend putting drive types in their own columns,” says Jerry Hoetger, director of product
management, RAID, at Xyratex. “Our analysis found that you can’t stack unlike devices because the differences
in vibration characteristics between 15,000rpm SAS drives and 7,200rpm SATA drives can cause performance
problems in a RAID scenario.” In fact, in internal testing, Xyratex discovered an 80% performance degradation
in a SATA drive that was positioned between two SAS drives.
However, Hoetger says that, assuming you follow vendors’ configuration guidelines, you shouldn’t have any
problems mixing SAS and SATA drives in the same enclosure. He estimates that 20% to 40% of Xyratex’s SAS
subsystems are configured with mixed drive types.
“Very few users are mixing drive types in the same enclosure, but that’s largely because most users aren’t using
SAS drives yet,” says Alan Johnson, director of marketing at Infortrend. “It’s much more common for users to
run mission-critical applications on a RAID array with SAS drives, and attach it to JBOD arrays with SATA
drives for applications such as virtual tape backup.”
Infortrend allows intermixing SAS and SATA drives with its controllers and enclosures as long as the drive
types are arranged in different vertical columns.
Adaptec’s Paniker notes that intermixing drive types in the same enclosure is very rare today, but that
the practice will ramp up as SAS drives become less expensive. (In the channel, SAS drives can be 3x to
5x more expensive than SATA drives.)
But intermixing drive types is not without controversy, and some vendors are vehemently against it. “You
should not mix SAS and SATA drives in the same enclosure,” states AMCC’s Cleland. “Some RAID vendors
are doing an injustice to SAS by selling SAS as a SATA controller and trying to push so-called ‘unified
storage.’ There aren’t many applications that will benefit from mixing SAS and SATA drives in the same
enclosure.” Clelend says that virtually none of AMCC’s controllers are configured with SAS and SATA drives.
“Right now, we won’t allow both drive types to be in the same array,” he notes.
