Is There Magic in SATA and SAS?

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SATAand SASare all the rage these day. They have captured the attention of the media, vendors and end users alike.

“SATA drives are gaining in popularity because of the speed benefits that they offer for data transfer,” said Ben Ginster, channel marketing manager for Gaithersburg, Md.-based Idealstor, a manufacturer of ejectable disk-to-disk backup solutions. “Local backups within a server are much faster than IDE, and network backups are fast as long as there is enough bandwidth and no network bottlenecks.”

But is SATA all it’s cracked up to be — and what about its SCSI equivalent, SAS? These technologies have definite advantages, but their rise to fame may be based on more than just their merits.

“Part of the popularity is from the media, as SATA seems to be the flavor of the day,” says Ginster. “A lot of customers will call and ask for SATA and have no idea why they want it, other than that is what they have been reading [about].”

Before we delve further, let’s examine what SATA and SAS are. The SATA acronym stands for Serial ATA — a method of attaching disk drives using a serial, rather than a parallel, connection. This standard uses only seven wires, as opposed to the 40-wire ribbon cable used in parallel ATA (PATA), is less susceptible to interference, and requires only a .25 V signal (rather than a 5V one).

SAS stands for Serial Attached SCSI (Small Computer System Interface), a protocol used to attach peripheral devices, primarily storage. It transfers data at a rate of 3 Gbps with less interference and over longer distances. Its costs are typically less than those of traditional SCSI, and it is compatible with SATA and older SCSI drives.


Don’t Worry, Be Happy

Jacob Farmer said he believes that end users shouldn’t worry about whether to use SATA or PATA. Farmer is CTO at Cambridge Computer, an integrator of data protection and storage networking solutions based in Waltham, Mass.

“One should not care what drive is inside as long as it meets the desired performance and reliability requirements. The magic is in the controllers and to some degree the disk cabinets and backplanes.”

— Jacob Farmer, CTO Cambridge Computer


“One should not care what drive is inside as long as it meets the desired performance and reliability requirements,” he says. “The magic is in the controllers and to some degree the disk cabinets and backplanes.”

He’s noticed that just about everyone has heard of SATA, and they believe it promises extraordinary interface bandwidth, which equates to overall performance. This, he says, is not necessarily the case. Farmer points out that companies like EMC and Network Appliance are still shipping plenty of products with PATA drives.

“This has created a lot of confusion among users who wonder whether they should stick with PATA or go with SATA,” says Farmer. “The short answer is that if the drives are sold inside of a cabinet and the cabinet is doing its job at the right price, it doesn’t matter what interface the drives use.”

SATA and PATA, after all, refer only to the way in which data is encoded and transported electronically. Parallel interfaces break a byte into 8 bits and send each bit down individual wires. A clock signal is sent down a separate wire. On the receiving end, with each stroke of the clock, the 8 bits are re-assembled into a byte and aggregated into a single fast signal. With serial interfaces, the bits all run down the same set of wires along with the clock signal.

Serial is a bit faster than parallel — up to 300 MBps compared to 133 MBps — at least in theory. But the speed of the bus does not automatically equate to actual throughput. In fact, many disk storage systems on the market today take little advantage of the additional bandwidth SATA promises.

Several companies, however, are harnessing SATA in innovative ways. IdealStor, for example, sells a disk-to-disk removable backup appliance called the FrankeNAS, a RAID device with ejectable SATA or PATA disks. It uses half of its 4U capacity as a RAID 5 array and has a capacity up to 2 TB. The other half of the unit is configured with up to four removable drive bays. The FrankeNAS ranges in price from $8,995 to $21,995.

“The ideal client for FrankeNAS is someone looking for NAS or RAID storage and offsite backup,” says Ginster. “Rather than purchasing NAS and tape, they can purchase the FrankeNAS and get a Windows 2003 Server with RAID storage and ejectable disk backup.”

For the moment, IdealStor’s sales are still heavily skewed toward PATA. However, Ginster expects SATA to dominate eventually.

Sepaton (so named for, “no tapes” spelled backwards) of Marlborough, Mass., is also implementing SATA in its S2100-ES2 virtual tape library (VTL) appliance. It operates like a physical tape library except it uses SATA RAID storage. The VTL has a capacity of 4.8 TB to 1 PB and speeds up the backup process to be as fast as 30 times that of tape and 24 times that of host-based disk-to-disk systems, according to Todd Phillips, a system engineer at Sepaton.

“Our appliance has a Fibre Channel [FC] backend and uses SATA disks for a good combination of reliability, performance and low-cost,” says Phillips. “FC works out at about two to three times the cost of SATA.”

Emulex of Costa Mesa, Calif., facilitates the replacement of FC disks with SATA in disk arrays. The company’s switching technology has been used inside FC arrays for years. Now, enterprises are choosing SAS and SATA inside to lower costs.

“The hierarchy is as follows: FC will remain for high speed and high reliability uses; SAS will populate the midrange; and SATA will take up the lower end.”

Todd Phillips, Sepaton systems’engineer


“They want to replace FC disks with low-cost, high-capacity SAS and SATA drives,” says Maziar Tamadon, director of product marketing at Emulex. “Instead of ripping out the controller and switch in the backplane, we build in technology to tunnel SAS and SATA over FC.”

He lays out the hierarchy as follows: FC will remain for high speed and high reliability uses; SAS will populate the midrange; and SATA will take up the lower end.


Mix and Match?


SATA and SAS drive connection technologies, then, are gaining ground as they provide higher data transfer rates, lower component costs, and more flexible architectures — though SAS is considerably behind SATA in terms of sales.

“SAS is essentially replacing SCSI, but it’ll take time for it to make its way into the enterprise, says Jay Krone, director of CLARiiON platforms marketing at EMC Corporation of Hopkinton, Mass. “SAS is being bought in servers right now, but remember that SATA drives are half the cost per GB.”

While EMC readies for the launch of its SAS line, it continues to turn out SATA products. The latest is the EMC CLARiiON AX150, which is priced from around $5,600.

Although SAS and SATA are compatible, Mike Karp of Enterprise Management Associates cautions against mixing different drives in the same enclosure.

“Only put drives of the same speed in the same bay,” he advises. “Devices spinning at different speeds cause vibration, and there is a danger that drives can unset or cause errors in reads and writes.”

Article courtesy of Server Watch

Drew Robb
Drew Robb
Drew Robb is a contributing writer for Datamation, Enterprise Storage Forum, eSecurity Planet, Channel Insider, and eWeek. He has been reporting on all areas of IT for more than 25 years. He has a degree from the University of Strathclyde UK (USUK), and lives in the Tampa Bay area of Florida.

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