Ultra320 SCSI - Page 2

Source: Adaptec

- SCSI Feature Sets

Mandatory features for Ultra320 SCSI as currently defined include:

• 320 megabyte per second transfer rate using DT data transfers
• 32-bit CRC
• Simple domain validation
• Backward compatibility
• Packetized transfers only
• A free-running clock
• Skew compensation
• A training pattern
• Transmitter precompensation with cutback

Optional features for Ultra320 SCSI currently include:
• AAF
• QAS
• Fairness
• AIP

- What's New

Ultra320 SCSI introduces additional technologies that will reduce overhead and improve performance. These changes will allow data to transfer safely and reliably at 320 MB/sec. Ultra320 SCSI includes the following key features:

• Double Transfer Speed: This doubles the transfer rate across the SCSI bus to a burst rate of 320 MB/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 Ultra160 SCSI, data is transferred in synchronous phase at 160 MB/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): This 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: This 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 turn-around delay (from DT data in to DT data out) is not incurred between each LQ and data packet.
  
• Flow Control: This allows the initiator to optimize its pre-fetching of data during writes and flushing of data FIFOs during reads. The target will indicate when the last packet of a data stream will be transferred which will allow the initiator to terminate the data pre-fetch or begin flushing data FIFOs sooner than was previously possible.

- 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. Later this year the drive data rates are expected to exceed 40MB/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 66 MHz. This allows for a maximum transfer rate of 533 MB/sec across a 64-bit PCI bus. With Ultra160 SCSI, two SCSI channels on a single device achieve a maximum transfer rate of 320 MB/sec leaving plenty of overhead before saturating the PCI bus. However, at 320 MB/sec, two SCSI channels can now achieve 640 MB/sec, which will saturate a 64-bit / 66MHz PCI bus. In addition to PCI-X doubling the performance of the host bus from 533 MB/sec to a maximum of 1066 MB/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 today’s applications.