The little cable that could
- By Joab Jackson
- Jul 12, 2005
Get out the red pens and mark up your enterprise storage RFPs. Serial technology will change the way you buy.
Who would have guessed that simply replacing a cable inside a computer system could cause profound changes in data management? Yet the industry's shift toward serial cables for connecting disk drives is destined to shake up the way agencies think of storage, prompting new types of networked solutions and redefining the roles of traditional storage arrays.
The bad news is IT administrators may have to scrap preconceived notions of how and where to store their growing mountains of digital information. The good news is the new serial technologies could help save agencies money, improve performance and streamline their storage infrastructures.
'There is a paradigm shift going on now. We're going from parallel technologies to a serial environment,' said David Deming, president of technology training company Solution Technology of Ben Lomond, Calif. He spoke last month at the Storage World Conference in Long Beach, Calif. 'What you will see is serial [advanced technology attachment devices] becoming the main interface in probably every computer environment you encounter today.'
Considering the breadth of computer environments in government use, that presents a significant shift agencies will want to exploit for their own gains.
Serial-attached storage 'fixes a lot of roadblocks,' in today's storage systems, Deming said.Building a better interface
Crack open a computer purchased a few years ago and you will find wide, bulky cables connecting the hard drive and optical disk. They use an interface called the advanced technology attachment. ATA is a parallel technology be- cause the cables have 16 side-by-side wires through which data is sent.
ATA has some well-known limitations. Using an ATA cable, you can attach a maximum of two devices to a motherboard, and ATA disks top out at a throughput of 133 megabytes/sec.
For workstations and servers that require more peripheral support and performance than ATA offers, system builders have long turned to another parallel adapter'the small computer system interface. SCSI can attach up to 15 devices to a single cable and reach a throughput of 320 megabytes/sec. It has long been considered a high-end direct-attached storage mechanism for mission-critical data. But SCSI can be a bother to set up, from assigning each drive a unique number to adding a terminator cap at the end of each SCSI ribbon.
Although both ATA and SCSI have been used exhaustively over the past 20 years, 'parallel technology is basically at an end,' said Martin Czekalski, vice president of the SCSI Trade Association and an interface architecture initiatives manager for hard drive manufacturer Maxtor Corp. of Shrewsbury, Mass.
Over the past few years, the American National Standards Institute has been overseeing upgrades to the ATA and SCSI interface standards. Both upgrades call for switching to serial interfaces. Unlike parallel interfaces, serial technology does not break the data into parallel conduits, but rather sends it serially along a single set of wires. The serial connector has seven pins rather than the 68 employed in parallel SCSI, or 40 in ATA. As a result, serial cables are much narrower than ribbon-like parallel cables.
Perhaps counterintuitively, the serial ap- proach promises higher throughputs de- spite having fewer wires.
'Think of a parallel bus as a multilane street. Things get slowed down by the intersections and traffic continually changing lanes,' Czekalski said. 'A serial bus would be like having two single lane 'drag strips' from one end of the city to the other without any intersections. You can now go full throttle, one car behind the next from one end to the other.'
Speaking more technically, Czekalski explained that parallel cables suffer from 'cross talk' between lines, as well as a serious mismatch of voltage requirements with the connecting chip. These attributes hinder designers' attempts to push the speed beyond today's limits. Serial connectors, however, can take advantage of improvements in chip technology to more quickly encode and decode data.The benefits of going serial
By most measures, serial-attached storage'whether serial ATA (SATA) or serial SCSI (SAS)'is a major improvement over previous technologies. The cables can transfer more data per second; they're smaller, which reduces the tangle of wires within storage arrays and servers; and they allow system builders to daisychain more devices to a computing node.
While all of these conveniences may make an IT administrator's daily life better, they also introduce fundamental changes in the way agency admins meet storage requirements.
Historically, storage has been a world of relatively discrete choices. Need mass storage with fast retrieval times? Buy a storage area network running over a blazing Fibre Channel link. Want to store terabytes of rarely accessed information on the cheap? Ring up a tape library. Looking for a little more storage for your server? String together a dozen hard drives using SCSI. Want more space but have minimal administration skills? A network attached storage unit is your best bet.
These days, serial-attached storage is blurring the choices, offering terabytes of space that can be much cheaper than Fibre Channel, more plentiful than direct-attached storage and faster than tape. And vendors are just starting to explore these possibilities. As they develop entirely new types of storage products, administrators will have to look carefully at the expanded range of options serial-attached storage will offer.
First-generation SATA products have only a slight edge in speed over their parallel brethren'about 150 megabytes/sec. Second-generation drives and peripherals, soon to be rolled out, should double that throughput.
The serial version of SCSI will start by offering around 300 megabytes/sec. and will double in the next few years, according to the SCSI Trade Association. In addition to speed gains in the cables themselves, SAS should free hard drive manufacturers to increase the response times of their disks, which is now limited to about 100 megabytes/second.
In addition to improving throughput, serial technologies can radically increase the number of devices that can be attached to a host. The SATA cable itself can only connect one device; you can attach up to 15 SATA cables to each host using a port multiplier. In a similar fashion, SAS can support up to 128 drives per host and can daisychain up to 16,000 devices with successive port expanders.
Serial also simplifies configuration. With SAS, administrators don't have to assign drives identification numbers or place a terminator at the end of each cable the way they do with traditional SCSI. And with SATA disks, administrators do not need to hand-set jumpers to designate the master-slave status of the drive. And since all the connections are point-to-point, they eliminate the potential bottlenecks and interruptions that plague multi-device busses.
Finally, there's another characteristic of serial technology that experts say could lead to interesting storage possibilities.
'By using SAS controllers, you can actually support SATA drives as well,' said Ron Engelbrecht, vice president and general manager of Engenio Information Technologies Inc. of Wichita, Kan. Engenio supplies components to companies selling storage systems, such as IBM Corp., Sun Microsystems Inc. and NCR Corp.'s Teradata division.
Such a feature could allow agencies to deploy single arrays that hold both faster performing (but more expensive) SAS disks and lower cost (but slightly slower) SATA disks. For example, if in the next year you're planning to buy two storage systems'an expensive one for holding mission-critical data and a cheaper one for less essential reference material'you might want to keep an eye out for a consolidated SAS/SATA offering. Such an array could be cheaper than two separate systems and offer more flexibility in altering the proportion of higher-priced, more reliable SAS disks to cheaper SATA ones.
But whatever your future storage plans, serial technologies will likely factor into them. 'SATA and SAS are going to happen,' said Deming. 'Whether you like or not, they're coming.'