SCSI: The next generation
The first part of a look at SCSI's future
By Galen Gruman, Information Age | Published: 08:00, 26 July 2005
That old standby storage standard, SCSI, is about to get a makeover. Two technology shifts, occurring in parallel and arriving this year, will change the kinds of disk drives enterprises use up and down their storage systems.
One transformation several years in the making is the move to a new interface -- SAS (serial attached SCSI) -- which provides faster, more flexible, and more reliable connections to drives. The new spec also permits the same drive enclosure to support SAS devices and lower-cost SATA drives.
At the same time, in part due to the advent of the smaller SAS interface, 2.5-inch enterprise-class drives will start to replace tried-and-true 3.5-inch models. In the long haul this will mean that datacentres will accommodate more storage without eating up more floor space. Smaller drives will also reduce power usage, speed data access, and increase the overall capacity of drive arrays.
At first the changes will be invisible, with new servers coming midyear with internal SAS drives instead of the traditional parallel SCSI ones. Lower-end drive arrays will start using SAS drives as well toward the end of the year. "SAS is now being qualified by the major OEMs," notes John Monroe, an analyst at Gartner. By 2008 to 2010 (estimates vary), all SCSI drives will be SAS drives.
"What IT will see is the continued trend of better capacity and performance at lower prices," says Greg Hartzog, head of storage infrastructure at consultancy Optimus Solutions.
Although SCSI technology is changing under the bonnet, enterprises don't have to worry about reworking their storage infrastructure to prepare for or manage the change. Because the drives use the same command set as previous SCSI drives, there's no change needed to the enterprise's storage architecture, as the SCSI command set and external interfaces remain unchanged. Also unchanged are the drives' head assemblies, the parts that store and read the data, notes Jay Krone, director of Clariion product marketing at EMC.
"There's not much that IT has to do," adds Franco Castaldini, enterprise storage product manager at Seagate Technologies. "They don't have to rip out their middleware or their storage management."
Older parallel SCSI drives are incompatible with SAS, so enterprises will have both sorts of SCSI devices in their DAS and SANs until the older devices are retired years from now. But that just means maintaining two types of replacement drives in case of failures and perhaps rearranging arrays to minimise having multiple cabinets, some of each type, in the same location, EMC's Krone says.
A smaller, more flexible interface
The move to SAS dramatically changes the connection between a drive and the backplane -- whether the drive connector on a server's motherboard or the host bus adapter within a drive array cabinet. The new connection is almost identical to the now-familiar SATA connector, and that's intentional.
SAS controllers work with both SAS and SATA drives because the cables are both physically and electrically the same. This will allow vendors to use the same power supplies, cases, and backplanes in all their products, reducing manufacturing costs and thus lowering prices to the enterprise, Optimus' Hartzog says.
"We'll have very high volumes [of SAS drives] because they will be used in both servers and arrays, so they'll be cheaper," says Craig Butler, disk storage product marketing manager at IBM.
In addition to the reduction in manufacturing costs that standardising the enclosures brings, the dual support for SAS and SATA drives by the SAS interface means enterprises can mix both types of drives in the same enclosure. That could help consolidate storage in one physical structure while supporting the typical functional split between the drives: using SAS for high-performance, high-transaction applications and SATA for low-performance, long-read applications such as archiving and streaming media.
But mixing the two drives together could cause problems if the drive enclosures aren't designed for it, notes Harry Mason, president of the SCSI Trade Association and director of industry marketing at chip-set maker LSI Logic. Due to their distinct disk rotations, SAS and SATA drives vibrate differently, causing the cabinet to shake. If they don't want the box walking across the floor, storage administrators who want to mix the two drive types in the same cabinet must ensure that the enclosure vendor has designed the cabinet to quell such vibrations.
In addition, some interfaces will be dual-use, whereas others will not. To prevent someone from plugging in an SAS drive to a SATA-only connector, the SAS cables have a plastic bump that prevents them from being inserted into anything but an SAS connector. SATA cables fit in both SATA and SAS connectors.
According to IBM's Butler, the SAS interface's support for both SATA and SAS drives "will make it easier to do tiered storage". A common interface means that enterprises can easily configure some drives for backup and archiving using SATA drives and others for transactional access using SAS drives, all with the same array cabinets and racks. The common interface will also make it cheaper for vendors to offer "cost-effective products up and down the line", he says, encouraging the use of tiered storage.