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Advances in Storage Connectivity

ATTO Ultra320 SCSI The Next GenerationA Technical Brief

Introduction:

Ultra320 SCSI represents the 7th

generationof SCSI technology, an I/O interface that iscommitted to increased performance whilemaintaining backward compatibility andlegacy support.

From its roots in 5 MB/sec. transfer rates,SCSI has evolved as the leading interface fordisk drive connections in high- performanceservers. With maximum data transfer rates of320 MB/sec., full backward compatibility with

older versions of SCSI protocols and severaladditional features to improve performanceand reliability, Ultra320 SCSI demonstratesthat SCSI technology is alive and well.

SCSI Evolution:

Over the past 20 years since its inception,SCSI technology has continued to evolve,providing faster data-transfer speeds,improved reliability and fewer cablinglimitations. The following chart illustrateshow SCSI has evolved.

Narrow Fast/Narrow Ultra Ultra/Wide Ultra2 Ultra3

Ultra160

Ultra320

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SCSI Advantages:

Backward compatible with olderversions of SCSI. Newer adapters will

negotiate to the lower speeds of legacydevices.

Minimal investment for upgradingtechnology. Older equipment may stillbe used in tandem with newerequipment. Upgrading does not requirea wholesale replacement ofinfrastructure.

It is still the fastest desktop and serverstorage technology. Ultra320 SCSI is2.5 times faster than Gigabit Ethernet,60% faster than 2-Gigabit FibreChannel, and 3.2 times faster than

standard Fibre Channel.

Whats New in Ultra320?

The Ultra3 SCSI specifications originallyincluded five features:

Double Transition Clocking Domain Validation

Cyclic Redundancy Check (CRC)

Packetization

Quick Arbitration Select (QAS)

Packetization and QAS were ultimatelyremoved as requirements, and thespecification was released as Ultra160SCSI.

Ultra320 SCSI supports all five Ultra3features and the following additional

features: Free-Running Clock

Read and Write Data Streaming

Flow Control

Training Pattern

Pre-Compensation

Asynchronous Information Protection(AIP)

Each of these additional features deliverskey benefits. The chart below summarizesthe features and benefits.

Features BenefitsDouble Transition ClockingIncreases the data line frequency to equal thatof the request signal, allowing sampling on boththe leading and trailing edges of the requestsignal. Clocking can be settable to ensurecompatibility with legacy devices

Increased performance, especially inenvironments that use extended transferlengths or have many SCSI devices on a singlebus

PacketizationCreates information units (IUs) comprisingcommands, data, status information and otherthings. These IUs are passed as synchronoustransfers

Maximizes bus utilization, minimizes commandoverhead and allows multiple commands to betransferred in a single connection

Quick Arbitration Select (QAS)

Arbitration, the process of devices negotiatingfor control of the bus, is a critical part of theSCSI specification. This process has built-inquiet times so that both fast and legacydevices have an opportunity to take control ofthe bus. Although fair, this process issomewhat inefficient. QAS speeds up thearbitration process by eliminating the bus freephase

When combined with Packetization, reducescommand overhead and maximizes busutilization

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Features BenefitsRead and Write Data StreamingMinimizes data transfer overhead by allowing a

target to send one data stream (LQ) packetfollowed by multiple data packets

Minimizes overhead of data transfers because

the target can send one data stream packetfollowed by multiple data packets

Flow ControlThe target indicates to the initiator when thelast packet of a data stream will be transferredso that the initiator can flush FIFOs andterminate pre-fetch sooner than previouslypossible. Basically, the target is warning theinitiator that the transfer is almost complete sothat it can prepare for the next transfer whilethe target completes the current transfer

Optimization, which allows faster transitionbetween transfers

Training Pattern

SCSI is a parallel bus technology. Therefore, itis dependent on signals being transmitted onparallel wires simultaneously. At higherspeeds, minute differences in wire lengths andtransmission characteristics could causeproblems. Training pattern testing measuresthese minute differences and compensates forthem

Helps measure and compensate for cablevariations

Pre-CompensationAlthough SCSI transfer speeds have changeddramatically over the past several generations,cable specifications have remained constant.

Higher speed (higher frequency, too) signalshave a greater potential for reflection anddistortion over distance. Pre-compensationtechniques slightly modify the SCSI signal toreduce the chance of these types of problems

Ensures a cleaner signals

Asynchronous Information Protection (AIP)Although most Ultra320 traffic is sentsynchronously and protected by CRC, someinformation is still sent asynchronously. AIPimplements CRC-level error checking onasynchronous traffic. This ensures end-to-enddata integrity

CRC class protection for asynchronous traffic

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Ultra320 Value Proposition

When implemented in a PCI-X environmentthat delivers 1 GB/sec., Ultra320 is apowerful storage technology. Today, all

forms of digital content from e-mail, video,film, and audio, to streaming video, andimaging are driving the unprecedentedgrowth in storage that pushes the I/Obandwidth and require more advanceinterfaces to handle the data transfer.

High Definition (HD) Digital Video (DV)applications require over 160 MB/sec.bandwidth per stream. Because most DVusers edit multiple streams simultaneously,the merits of Ultra320 technology are easyto see. Data mining and real-time

transaction applications will benefit from thenew QAS and Flow Control features,allowing servers to search more disks forinformation in a shorter period of time.

Ultra320 SCSI is more than a simple speedimprovement. It provides a rich feature setfor improved performance, reliability andefficiency that will allow SCSI to retain itsoverwhelming market share in theperformance storage market.

Definitions

Double Transition Clocking SCSItransactions (data transfers) include requestsignals and data lines. Ultra2 and earlier

SCSI implementations used single transitionclocking whereby the frequency of the datalines was half of the request signal. Thisallowed data sampling only on the leadingedge of the request signal. Doubletransition clocking increases the data-linefrequency to equal that of the request signal,allowing sampling on both the leading andtrailing edges of the request signal.Clocking can be settable to ensurecompatibility with legacy devices.

Domain Validation Before sending data,

domain validation is performed to verify thatthe physical connection (cables, connectors,targets, etc.) is capable of handling thenegotiated transfer speed. Although a targetand initiator may indicate that they are ableto sustain Ultra320 transfer rates, duringDomain Validation a series of tests isperformed to ensure the sustainability andreliability of that connection. If it isdetermined that Ultra320 speeds are notfeasible, a slower speed is enforced.

Cyclic Redundancy Check (CRC)

verifies that the data sent matches with thereceived data. CRC is more robust than thesimple byte parity checks used in pre-Ultra160 SCSI. Increased data rates andlonger cable lengths, among other factors,have resulted in the potential for increasederror rates, necessitating the use of CRC.

Packetization Later generations of SCSIare more adversely impacted by an increasein overhead since traffic is negotiated atsynchronous and asynchronous speeds.Packetization creates information units (IUs)comprising commands, data, status

information and other things. These IUs arepassed as synchronous transfers, reducingoverhead and improving overall efficiency.

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Quick Arbitration Select (QAS) Arbitration, the process of devicesnegotiating for control of the bus, is a criticalpart of the SCSI specification. This processhas built-in quiet times so that both fast

and legacy devices have an opportunity totake control of the bus. Although fair, thisprocess is somewhat inefficient. QASspeeds up the arbitration process byeliminating the bus free phase. Combinedwith packetization, this significantly improvesbus efficiency.

LVD Low Voltage Differential. It is theSCSI signaling method that combines thebenefits of HVD and Single-endedtechnologies, allowing longer cablingconfigurations (25 meters point-to-point, 12

meters with multiple devices) whileconsuming less power than HVDtechnology.

HVD High Voltage Differential. It uses twowires, transmitting a signal on one and itsinverse on the other. At the receiving end,the difference between the two signals ismeasured and interpreted. Noise on the buswill affect both the signal and its inverseequally, so the difference between the twolines will remain the same and the noise cannot be misread as a signal.

Single-Ended An electrical signal protocolthat transmits information through changesin voltage. Single-ended SCSI usesstandard TTL signal and ground pairs totransmit information over the SCSI bus.

Free-Running Clock Allows use of ahigher frequency clock with legacy cablesand infrastructure. This is accomplishedthrough the use of a Data Enable signal inaddition to the Request signal and Datalines.

Read and Write Data Streaming Minimizes data transfer overhead byallowing a target to send one data stream(LQ) packet followed by multiple datapackets.

Flow Control The target indicates to theinitiator when the last packet of a datastream will be transferred so that the initiatorcan flush FIFOs and terminate pre-fetchsooner than previously possible. Basically,

the target is warning the initiator that thetransfer is almost complete so that it canprepare for the next transfer while the targetcompletes the current transfer.

Training Pattern SCSI is a parallel bustechnology. Therefore, it is dependent onsignals being transmitted on parallel wiressimultaneously. At higher speeds, minutedifferences in wire lengths and transmissioncharacteristics could cause problems.Training pattern testing measures theseminute differences and compensates for

them.

Pre-Compensation Although SCSItransfer speeds have changed dramaticallyover the past several generations, cablespecifications have remained constant.Higher speed (higher frequency, too) signalshave a greater potential for reflection anddistortion over distance. Pre-compensationtechniques slightly modify the SCSI signal toreduce the chance of these types ofproblems.

Asynchronous Information Protection(AIP) Although most Ultra320 traffic is sentsynchronously and protected by CRC, someinformation is still sent asynchronously. AIPimplements CRC-level error checking onasynchronous traffic. This ensures end-to-end data integrity.