Storage suppliers collaborate on new SCSI technology

Although serial-attached small computer system interface (SCSI) disc drives and interfaces are not yet a reality, five...

Although serial attached small computer system interface (SCSI) disc drives and interfaces are not yet a reality, five manufacturers - Adaptec, Fujitsu, Hitachi, Maxtor and Seagate - have announced they will ensure their serial attached SCSI products will be interoperable.

SCSI is an electronic computer standard that allows computers to communicate with peripheral devices such as disc drives, CD-Rom drives, printers, tape drives and scanners. SCSI interfaces are currently deployed in a parallel fashion, but this new industry initiative is aims to deploy SCSI using a serial-attached format.

Linus Wong, director of strategic marketing, storage solutions division at Adaptec, said serial-attached SCSI is a natural evolution from its currently deployed counterpart, parallel SCSI, and will be on the market in about 12 months.

"SCSI has doubled in performance every couple of years in the past five to 10 years. We are on our seventh generation of SCSI at 320 megabits per second," Wong explained. "But each of those transitions have got more and more difficult. Serial technologies in general allow you to break down that barrier and make a leap to that next generation technology."

Wong also sits on the board of the industry body the SCSI Trade Association, which drives positioning and standards around SCSI. He said the association has abandoned plans for what would have been the next generation of parallel SCSI - Ultra-640 - and is now focusing on serial-attached SCSI which would initially reach speeds of 1.5gbps.

"In the parallel technology you’re sending multiple bits down the wire at the same time, and they all have to get there at the same time and be clocked at the same time. So as you go faster it becomes almost impossible to get all those bits to line up," Wong explained.

"At the current generation of SCSI there are six bits on the wire at the same time. So to get those to line up and arrive at the other side of the cable at exactly at the same time is very difficult."

But with serial-attached SCSI one bit is sent at a time, so there is no need to worry about clocking or skew compensation. That is why Wong says these faster speeds can be attained. In addition to the higher bandwidth, serial-attached SCSI devices will also be able to transmit data over large geographic distances.

Parallel SCSI can only address a limited number of devices and has performance as well as distance limitations plus large connectors that make it unsuitable for some dense computing environments.

Wong says serial-attached SCSI disc drives can be attached to many devices, while parallel SCSI drives, because they share a bus, can only be attached to about 15 devices.

"Because [serial-attached SCSI] is a point-to-point technology, each device has a dedicated link, you can fan out to many more devices," Wong said. "So, from a spec point of view it is tens of thousands, but for all practical purpose the limit is probably 120 to 256 devices."

One drawback is there will be no compatibility with serial-attached SCSI devices to parallel SCSI drives. However, Wong said bridges will be included that can span serial-attached SCSI and parallel SCSI environments when the technology hits the streets.

This means a user would have to invest in new hardware to make the transition from parallel to serial-attached SCSI, but Wong said the software needed to administer these two environments would be the same. Only minor adjustments in modifying parallel SCSI protocols to serial-attached SCSI protocols would be required.

In addition, from a standards perspective, Wong said the T10 Technical Committee, part of the InterNational Committee on Information Technology Standards, which has defined standards for parallel SCSI, is also working on developing standards for serial-attached SCSI.

Another benefit, Wong said, is that serial-attached SCSI interface supports serial-attached SCSI disc drives and Serial Advanced Technology Attachment (Sata) disc drives.

Like serial-attached SCSI, Sata is a relatively new technology. Wong says the first products are just starting to ship now and is slated to replace parallel ATA disc drives such as integrated drive electronics (IDE), enhanced IDE (EIDE) and UDMA 66, usually seen on the desktop environment.

With Sata, users can get faster speeds than with parallel ATA, and, enterprises are showing interest in deploying them in a server environment in lieu of SCSI drives for storing reference data because Advanced Technology Attachment drives are less expensive.

Unlike transactional data, which is constantly being accessed and requires updating in real-time, reference data does not change and is not accessed often so, Wong said, enterprises do not mind compromising speed for cost-savings in this instance.

While Advanced Technology Attachment drives do not perform as well as SCSI drives, Wong said this could be partially compensated for by Redundant Array of Independent Discs (Raid) software and Sata disc drives are ideal for storing reference data.

"[The industry is] positioning those as an alternative to tape drives, or at least a staging between tape and disc because it is less expensive," said Alan Freedman, research manager, infrastructure hardware with IDC Canada.

"You don’t get quite the performance [as SCSI] although I don’t think there is a huge decrease anymore. But, especially for things like fixed content which is going to change a lot and which you don’t need to recall all that often, this is a good option."

Wong said the interoperability of serial-attached SCSI interfaces with serial-attached SCSI disc drives and Sata disc drives would mean enterprises, OEMs and resellers could be flexible in their storage deployments.

Freedman said this is yet another option for people to prioritise their data storage needs.

"It gives them the chance to create a storage infrastructure that is properly aligned with their business initiatives and that enables them to save money while getting the performance that is necessary," Freedman said. "It is not always about getting top performance, it is about getting the performance that you need."

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