In step with the times: new features speed Ultra320 SCSI - From the SCSI Expert

Computer Technology Review, August, 2002

Ultra320 SCSI is the current step the SCSI evolution. With the introduction of Ultra160 SCSI, three key technologies were introduced and include dual-edge clocking, domain validation, and cyclic redundancy check (CRC). Now Ultra320 SCSI introduces additional technologies that require for the first time, protocol changes that will reduce overhead and improves performance. These changes will allow data to transfer safely and reliably at 320MB/sec. Ultra32O SCSI includes the following key features:

Double transfer speed: Doubles the transfer rate across the SCSI bus to a burst rate of 320MB/sec, allowing higher transfer rates across the SCSI bus and increasing the disk drive saturation point. This results in increased performance, especially in environments that use extended transfer lengths or have many devices on a single bus.

Packetized SCSI: This includes support for packet protocol. Packetized devices decrease command overhead: by transferring commands, data, and status using DT (dual transition) data phases instead of slower asynchronous phases. This improves performance by maximizing bus utilization and minimizing command overhead. Furthermore, packet protocol also enables multiple commands to be transferred in a single connection. In Ultra 160 SCSI, data is transferred in synchronous phase at 160MB/sec, while the command . and status phases are still transferring at slower asynchronous phases and limited to a single transfer per connection.

Quick arbitration and selection (QAS): Reduces the overhead of control release on the SCSI bus from one device to another. This improvement reduces command overhead and maximizes bus utilization.

Read and write data streaming: Minimizes the overhead of data transfer by allowing the target to send one data stream LUN Q-TAG (LQ) packet, followed by multiple data packets. In a non-streaming transfer, there is one data LQ packet for each data packet. Write data streaming performance is also increased because the bus turnaround delay (from DT data in to DT data out) is not incurred between each LQ and data packet.

Ultra320 SCSI Lines Up With PCI-X

Faster I/O performance will saturate the PCI bus, therefore, most host implementations are tied to PCI-X. Disk drive media rates continue to increase. This year the drive data rates are expected to exceed 80MB/sec. SCSI will need to jump past Ultra160 SCSI in order to support sustained throughput from the average number of drives in a server (four). Under standard PCI the host bus has a maximum speed of 66MHz. This allows for a maximum transfer rate of 53MB/sec across a 64-bit PCI bus. With Ultra160 SCSI, two SCSI channels on a single device achieve a maximum transfer rate of 320MB/sec, leaving plenty of overhead before saturating the PCI bus. However, at 320MB/sec, two SCSI channels can now achieve 640MB/sec, which will saturate a 64-bit/66MHz PCI bus. In addition to PCI-X doubling the performance of the host bus from 533MB/sec to a maximum of 1,066MB/sec, there are protocol improvements so that efficiency of the bus is improved over PCI. Together PCI-X and Ultra320 SCSI provide the bandwidth necessary for t oday's applications.

SCSI enters its 21st anniversary by ushering in a new technology. Ultra320 SCSI is sure to add to the existing legacy of past SCSI technologies. SCSI has come a long way from its original 5MB/sec transfer rate. At 320MB/sec, Ultra320 SCSI is only the latest in SCSI evolution. As technology continues to move into the 21st century, the industry can continue to look forward to new and faster SCSI technology.

With new technologies, such as the packetized feature, SCSI will continue to deliver performance safely and reliably for generations to come. As performance continues to grow, so will the applications that can take. full advantage of greater I/O performance. PCI-X accelerates performance across the host bus to 1,066MB/sec and Ultra320 SCSI is there to take full advantage of this available bandwidth. And as always, SCSI maintains its backward compatibility allowing customers to protect their investment while concurrently giving them the ability to grow as their needs increase. No other I/O technology can provide these advantages. SCSI continues to increase its performance, features, enhancements and market share. Ultra320 SCSI is the newest example of SCSI's continued commitment to providing, the industry with the I/O bandwidth necessary for an increasing number of performance-hungry applications.

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