Opening the Serial (Storage) Box

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It’s always fun, as a kid, being the first in the family to open a new box of cereal and dig a hand in searching for the hidden prize. You never knew whether it would be that wonderful Grand Prize they advertised on TV, or a lousy two-cent plastic trinket.

Now as an adult, you no longer have that moment of breathless anticipation. When you open up a server box, you know exactly what will be inside. ATA drives in ATA boxes and SCSI drives in SCSI boxes.

Those days, however, will soon be over. In October, Maxtor Corp. and LSI Logic Corp. (both based in Milpitas, Calif.) conducted the first public demonstration of data exchange between Serial ATA (SATA) and Serial Attached SCSI (SAS) hard drives using SAS controllers and expanders.

In this article we examine the significance of this development and what it means to IT.

A Tale of Two Standards

Other than a lot more bytes for your buck, the biggest development in storage over the past few years has been in the area of virtualization. It enabled the creation of architectures to optimize the benefits of direct-attached storage, storage area networks and network-attached storage.

While that improved storage integration, hardware segregation continued to exist. Companies, after all, still rely on two types of disks. ATA drives provide the highest capacity and lowest price per gigabyte — ideal for desktops. The higher-speed, higher- performance and higher-priced SCSI drives, meanwhile, hosted mission-critical applications in the data center.

If that arrangement had lasted, there would be no conflict. Recently, however, ATA drives have migrated into the data center for “nearline” applications which can tolerate lower levels of I/O performance and reliability.

“New applications are emerging that use ATA disk drives in the data center,” says Joe DeRosa, senior director of marketing in Maxtor’s Server Product Group. “These nearline applications do not require the high IOPs [Inputs/Outputs Per Second] or levels of availability that enterprise drives provide, but they do require very large amounts of inexpensive capacity.”

This includes functions such as performing disk-to-disk backups, archiving e-mail, storing reference data and streaming media — items which consume a large amount of disk space but which aren’t frequently accessed.

While this allowed an enterprise to make the best use of both types of drives, the arrangement was far from ideal. These drives had incompatible interfaces, meaning that IT staff had to support two different architectures and maintain separate servers for the two types of drives.

Getting Off the Bus

Underlying this incompatibility was the fact that both ATA and SCSI used a parallel bus interface. Even if a company was only going to use one type of drive or the other, parallel connections had to go in order to raise performance levels. To begin with, the 40-conductor ribbon cables used by parallel ATA are too bulky and cumbersome for today’s rack-dense servers. Parallel transmissions are also too slow and subject to signal errors from crosstalk.

ATA was the first to step off the serial bus. In August 2001, the Serial ATA Working Group (www.serialata.org) issued the Serial ATA (SATA)1.0 specification which got rid of the ribbon cable and boosted data throughputs to 1.5 Gigabits per second. It followed this up with SATA 2.0 which doubled the data rate.

SATA 2.0 also added command queuing which further raises the efficiency of traffic management by as much as 30%. The first SATA drives started shipping in the first quarter of 2003.

Three months after the release of SATA 1.0, the Serial Attached SCSI Working Group formed. In addition to moving SCSI drives over to a serial standard, it also decided to share the same physical and electrical interface used by SATA, so that the drives could interoperate. This allows the customer to select a drive based on which best meets its operational needs, rather than having to select one which matches existing hardware.

The SAS protocol involves more than just the disk drives and their connectors. It also incorporates Host Bus Adapters (HBAs), RAID Controllers, drive enclosures and expanders. Expanders are virtual circuit switches that sit between the SAS drives and the RAID controllers. Each expander can address up to 64 devices and allow up to 4096 devices in an SAS domain. Point-to-point connections run at 3 Gigabits per second.

In addition to the hardware compatibility, SAS also incorporates protocols which allow SATA drives to operate in a SAS domain. SAS includes three different protocol types. Serial SCSI protocol for sending SCSI commands, SCSI Management Protocol for management data, and Serial ATA Tunneled Protocol for sending SATA commands.

So, what does all this mean for IT managers? No, not just the thrill of opening up a server enclosure and finding an unexpected type of drive inside. It actually equates to greater performance, lower costs and simplified management.

“SAS offers the redundancy and performance features of fibre channel with the flexibility to support SATA hard disk drives,” says DeRosa.

Among the benefits, SAS provides:

The ability to upgrade from SATA to SAS drives without having to buy a new system.

Seamlessly move infrequently accessed files to near line storage, reducing the need to buy more SCSI drives.

Reduction in replacement hardware inventory.

Lower staff training costs since they just need to know one set of protocols.

Greater reliability since SAS drives have dual ports.

And all of this is at a much lower cost than a fibre channel arbitrated loop.

Okay, so all that sounds fine, but does it actually work? Well, October’s demonstration of data exchange involved all three of the SAS protocols discussed above, together with uses of the SATA protocol.

According to David Steel, LSI’s director of product planning and management, this was the first demonstration of all four protocols working together simultaneously.

With the testing successfully completed — not just by Maxtor, but by Seagate, Adaptec and other vendors — manufacturers are now readying SAS products for the market. The SAS 1.0 standard was finalized in November and products will start arriving in the second half of 2004. For more information on the SAS standard and products, visit the SCSI Trade Association Web site.