The popularity of SCSI is built on rigorous protocol specifications that define how SCSI host adapters, peripheral devices, and connectivity cables work together. SCSI has long provided – even in its earlier versions in the 1980s – a fast, reliable, and consistent interface to connect internal and external peripheral devices such as disk drives, tapes, printers, scanners, CDs, and DVDs to host computers. In fact, manufacturers of many disparate classes of devices have focused on SCSI specifications for seamless, standards-based connectivity to multiple platforms.
The latest version of SCSI, Ultra160, maintains connectivity to as many as 15 devices and supports a maximum bus length of 12 metres, but doubles the data transfer rate of its predecessor, Wide Ultra2 SCSI, to 160MBps (mega bytes per second). Ultra160 SCSI is currently available in products from major vendors such as Adaptec Inc., IBM Corp., LSI Logic Corp., Quantum-Maxtor, and Seagate Technology Inc.
In addition to its staggering transfer rate, Ultra160 offers unprecedented flexibility in mixing SCSI devices with different capabilities because it can automatically negotiate a lower connection speed for a device. Should a device’s initial connection setting fail, the controller will simply communicate with it at a slower speed without affecting others in the chain.
This obvious management simplification keeps a device working when the optimal transfer rate cannot be maintained, and makes the previously inflexible SCSI more forgiving of configuration errors or minor component failures. And, to detect possible data-bit loss, Ultra160 introduces an error-control mechanism called: “cyclic redundancy check” that replaces the simpler parity-check mechanism used in previous versions.
In summary, Ultra160 shortens the transfer rate gap between SCSI and Fibre Channel, improves manageability, and adds better error-detection features to SCSI protocol, preserving the same LVD (low voltage differential) connectors, cables, and terminators of Ultra2. However exciting, the new features of Ultra160 aren’t the only advances in SCSI on the horizon. New SCSI standards promise a further increase in data-transfer performance to 320MBps in 2002, and another overwhelming jump to 640MBps expected for 2003.
Although Ultra320, which has already been demonstrated in a RAID configuration by Adaptec Inc., pushes the transfer rate well beyond the 200MBps delivered by 2Gbs Fibre, this edge in performance will not automatically grant SCSI a place in networked storage. To be successful in that area, SCSI needs to overcome its distance limitation, where it probably pales the most in comparison with Fibre Channel.
Current SCSI implementations can take advantage of bridges to connect SCSI devices to a Fibre Channel network. Nevertheless a more affordable connectivity medium is still needed, such as GbE (Gigabit Ethernet), as is a more familiar protocol, such as TCP/IP, for use in storage networks. The convergence of SCSI, GbE, and TCP/IP is producing a new standard, called iSCSI, that promises to allow companies to leverage their existing investments in those technologies for less expensive and less esoteric storage networks.
Today’s SCSI seems to have little in common with its earliest predecessor, which was limited to 8-bit connectivity and data transfers of 5MBps. Nevertheless, an Ultra160 controller can still connect to one of those old devices. Maintaining backward-compatibility as it evolves to meet future needs is no doubt the main reason for the popularity and longevity of the SCSI standard. With that in mind, the best of SCSI is yet to come. Stay tuned.
Adaptec 39160 SCSI controller
We put Ultra160 SCSI to work in an outdated Dell P2200 server, with an Ultra Wide SCSI controller and three Seagate Barracuda drives (each with a 4GB capacity) that were begging to be replaced. For our new SCSI controller, we chose the Adaptec 39160.
The 39160 has two separate channels, each capable of connecting as many as 15 devices and moving data at rates as fast as 160MBps. Each channel has one external and one internal connector for 16-bit devices. A third connector on one of the channels allows you to connect legacy 8-bit devices.
To replace our 4GB Seagate drives, we installed three Maxtor Corp. Atlas 10K III drives in 36GB configurations. These drives, which are also available in 18GB and 73GB capacities, spin at 10,000 rpm, boast exceptionally fast seek times, and support transfers of 160MBps.
To make our test more interesting, we added one of the old Seagate drives to the unused channel of the Adaptec 39160 controller. After booting, we invoked SCSISelect, Adaptec’s configuration tool, and verified that all four drives were properly recognized. The 39160 also correctly negotiated transfer speeds of 160MBps for the new drives and 40MBps for the old one.
Considering that the Maxtor drives sell for less than US$600 each and the Adaptec 39160 costs $380, we were able to increase our disk space to roughly 100GB and improve our server performance significantly at a moderate cost. Moreover, we had consolidated new and legacy devices under a common controller and had room to connect more. Ultra160 SCSI was an elegant way to solve our performance and capacity problem. It can work for your company as well.
THE BOTTOM LINE
Executive Summary: The newest version of SCSI, Ultra160, offers performance comparable to Fibre Channel while preserving compatibility with previous versions, improving manageability at a moderate cost.
Test Center Perspective: Forthcoming versions of SCSI and related technologies promise further increases in performance and integration with TCP/IP to form Ethernet-based storage networks. Companies should evaluate Ultra160 to address current needs, and consider future evolutions in their long-term plans.
Mario Apicella covers storage technologies for the InfoWorld Test Center. Please address your comments and questions to [email protected]
Prices listed are in US currency.