SCSI: An Enterprise Foundation Harry MasonLSI Logic Corp.

O riginating as a general-purpose interface stan-dard, SCSI debuted with rich capabilities,including multi-initiator support, sophisti-

cated error management, out-of-box connectivity,and support for a wide variety of peripherals. SCSI’simmediate popularity with Apple Macintosh usersspawned a market for various peripheral devicesconnected outside the desktop box.

Early developmentsAlthough the standard’s founders knew SCSI must

evolve to enjoy continued success, the enterprise-classcapabilities they envisioned had yet to be exploited.Instead, a rather small enterprise market, and variousretail and database applications, focused on enter-prise-oriented companies like NCR, which sought tohelp multi-initiator applications flourish.

0018-9162/02/$17.00 © 2002 IEEE48 Computer

Surveying Today’s Most Popular Storage Interfaces

S torage interfaces have been a necessarycomponent of computer systems sincecomputing’s inception. At a basic level, astorage interface functions like anygeneric interface, defining the boundary

between two dissimilar surfaces or systems. In acomputer system, a storage interface defines boththe boundaries between storage devices—such ashard drives, tape drives, or similar media—andhow those dissimilar computing resources engage

one another to work as a coherent system. Today’s storage interface arena consists of

diverse industry standards combined with R&Dinvestments from major industry players who con-tinue to aid in the evolution of these numeroustechnologies. Although SCSI—the Small ComputerSystem Interface—is probably the most pivotalstandard in use today, other crucial storage proto-cols include Fibre Channel, IEEE 1394, Serial ATA,and iSCSI.

Backed by strong industry support, SCSI, Fibre Channel, IEEE 1394, Serial ATA, and iSCSI provide the technologies to meet IT’s diverse storage interface needs.

Michael T.LoBueLoBue & MajdalanyManagement Group

C O V E R F E A T U R E

Fueled by low-cost controller devices, a maturingsoftware base, and clear market value, SCSI com-manded a substantial presence in both the desktopand server markets. Using SCSI allowed adding drives with faster spindle speeds to workstations,adding optical drives and scanners to PCs, and usingtape to maintain streaming capabilities in servers.

SCSI made systems simpler. Functional upgradesto the volume PC market made SCSI an attractive,profitable option that encouraged a steady stream ofongoing market investments—ranging from siliconsuppliers to cable, connector, and terminator sources.

Enterprise adoptionSCSI’s low-cost mission-critical capabilities made

it a perfect choice for the vast majority of enter-prise-class systems. Manufacturers who followed

December 2002 49

Table 1. Serial Attached SCSI features and benefits.

Feature Benefits

Leverages industry standards Fast time to marketImproved interoperabilityPerformance roadmap to 6.0 Gbps

Allows coexistence with SATA Flexible price-performance pointsdisk drives Single backplane designPoint-to-point architecture Ease of scalabilitysupports more than 128 devices Flexible topologiesThinner cables and fewer signals Improved cable routing, airflow, and coolingSmaller connectors Meets requirement for dual-port enterprise

2.5-inch hard disk drives

the standard focused on improving enterprise capa-bilities across their product lines, from standardhigh-volume servers to the most sophisticated suchas Symmetrix.

Eliminating the cabling between devices withina storage enclosure provides a key example ofadapting to the market’s demands. By establishingbackplane standards for “hot plugging” devicesand instituting a move to low-voltage differentialsignaling schemes, SCSI greatly improved systemperformance while maintaining the connectionlengths required between devices.

Even with such significant changes, SCSI’sunique shared distributed bus structure let olderdevices coexist with the latest generation of SCSIperipherals. The creation of SCSI Expander com-ponents further supported the use of legacy periph-eral devices and their accompanying software.

Broadening SCSI’s market appealAlthough SCSI adequately addressed system

connectivity needs at the drive-interface level, themarket demanded greater connectivity betweensystems and storage subsystems—especially inlarger systems. Managing large volumes of storageas a unified resource required connections thatallowed for increased distances between boxes,higher degrees of scalability, and failover and load-balancing schemes to govern these connections.

Fibre Channel effectively met the new demandsof mission-critical application environments byusing the SCSI protocol’s enterprise-proven logi-cal-connection capabilities. Carrying the logicalcommand descriptor block structure of SCSI for-ward proved a critical step in serving enterprise-class environments. In effect, Fibre Channelexpanded SCSI’s influence into storage area net-works, making the logical SCSI interface essentialto Fibre Channel’s market position.

Fibre Channel’s success proved SCSI’s value andmarked its place as an essential foundation for sub-sequent enterprise storage initiatives, includingStorage over IP, iSCSI, and InfiniBand.

Ultra 320 SCSI and beyond With the current generation of Ultra 320 SCSI

components, controllers, and drives, systems cansustain performance levels in excess of 100,000I/Os per second while working smoothly with cus-tomers’ existing hardware. New generations ofSCSI devices can consistently coexist with previ-ous generations, thus preserving 20-plus years ofenterprise-proven SCSI software.

With the proven capabilities of Ultra 320 SCSI and

Ultra 640 SCSI’s greater promise, parallel SCSI willlikely remain an essential factor in enterprise systemsand device connection schemes for years to come.However, as with any parallel connection scheme,moving these interfaces forward becomes measur-ably more difficult with each generation. Supportingsmaller, power-efficient form factors at a drive datarate doubling every 2.2 years—while maintainingsignal integrity for such high-availability systems— presents a substantial challenge.

Serial Attached SCSIWith an eye to the future, and mindful of their col-

lective enterprise legacy, several leading companieshave embraced a new storage initiative—SerialAttached SCSI. Table 1 lists Serial Attached SCSI’sfeatures and benefits. Standardization on this newinterface has the potential to bring together the bestof parallel SCSI, Fibre Channel, and the emergingSerial ATA.

Just as parallel SCSI embraced the challenges ofhot plugging and improved signaling, SerialAttached SCSI is now responding to the challengesof tomorrow’s mission-critical application envi-ronments, which include

• smaller form factors,• greater addressability with support for higher

spindle counts,• greater flexibility for in-box and near-box

cabling schemes,• increased reliability with dual-porting capa-

bilities, and• unprecedented customer choice.

Complementing the SATA interface development’sefforts, Serial Attached SCSI lets the customer chooseamong SATA drives competitively priced for a cost-driven volume market. These drives deliver aconnection scheme that accepts high-performance,robust storage devices capable of serving the mostdemanding enterprise applications. By employingSATA physical signaling and mating schemes,embracing Fibre Channel’s packet-based approachto switched SCSI connections, and preserving the

T he problems IT managers face—the explosionof storage needs, proliferation of rich-contentmedia, and Internet use—are quickly making

older, direct-attached storage technology obsolete.A new paradigm in storage has arisen in the pastfew years to replace DAS: storage area networks(SANs). Fibre Channel, the enabling technologybehind this SAN revolution, has blazed a trail fornew developments that benefit the storage space.

The SCSI protocol provides the basis for mostenterprise-level DAS. SCSI ensures that systemsstore data efficiently and correctly, operating at theblock level and making access for both reading andwriting data extremely efficient. This approach dif-fers from file-level access to data by requiring significant overhead and operating-system inter-action.

SCSI’s benefits come at a price, however: Only15 devices can be attached to a SCSI controller, andthe distance between controller and storage can-not exceed a few meters. These limitations makeadding storage to a system both difficult and disruptive.

Fulfilling the needs of enterprise-level companiesthus required a new protocol that would meet sev-eral key requirements:

• Allow SCSI use. Fibre Channel’s developersdesigned it to operate as a SCSI protocolsuperset, but also to let other, higher-level pro-

tocols access the underlying transport layer,including TCP/IP over Fibre Channel.

• Increase device count per controller. FibreChannel introduces an arbitrated loop,increasing the number of devices available percontroller to 126. Fibre Channel’s fabric capa-bilities increase the total number of devicesaddressable in a Fibre Channel SAN to morethan 16 million.

• Increase distance between devices. FibreChannel allows copper connections out to sev-eral meters, basic optical connections to 500meters, and long-wave connections to 10 kilo-meters. Some implementations allow directconnections to distances of 80 kilometers.These distances allow direct data center con-nections via native fiber channels. FibreChannel also includes the FCIP protocol,which allows long-haul connections world-wide. Islands of SANs can thus be connectedanywhere across the globe.

• Allow both fiber and copper connections. Fromits inception, Fibre Channel’s developersdesigned it to work with both copper andoptics. This dual capability allows inexpensivecopper connections for intracabinet, intrachas-sis, and very-short-haul distances, while opti-cal connections permit longer-haul distances inan EMI-safe context. An urban legend main-tains that Fibre’s spelling actually reflects the

50 Computer

logical SCSI protocol itself, Serial AttachedSCSI promises new interface capabilities forthe enterprise.

Serial Attached SCSI serves the enterpriseby leveraging a common mating scheme,enclosure, and infrastructure. This flexibleconnection scheme presents opportunities forbuilding products that could ship as soon as2004.

SCSI’s bright futureSCSI has fundamentally influenced the

enterprise market’s evolution. Every new stor-age initiative demonstrates its significance in theenterprise. Serial Attached SCSI propels this influ-ence well into the future by maintaining compati-bility with SCSI’s rich and successful legacy. Thisprotocol offers a serial point-to-point connection

scheme that lets OEMs, system integrators, and cus-tomers choose how best to develop their storageapplication environment.

The industry’s commitment to furthering SCSI’sinterface capabilities has made it a lasting founda-tion for the enterprise. The ANSI-accredited T10Technical Committee (http://www.t10.org) is atwork on this industry-standard development ini-tiative. Meanwhile, the SCSI Trade Association,STA (http://www.scsita.org), holds the responsibil-ity for the interface’s business development.

Harry Mason is the director of industry marketingat LSI Logic Corp. Mason received an MS in elec-trical engineering from the University of Missouri.He has been the president of the SCSI Trade Asso-ciation for five of the past six years. Contact himat hmason@lsil.com.

Fibre Channel DeliversThomas Hammond-DoelVixel

SCSI’s serial point-to-point connectionscheme lets userschoose how best to develop their

storage applicationenvironment.

December 2002 51

conjugation of the words “fiber” and “wire,”rather than the European spelling of “fiber.”

• Provide nondisruptive scaling. The need toexpand a storage system or SAN without hav-ing to reconfigure or power down a system ledto the requirement for hot-plugging devices.This requirement manifested itself in hot-plug-gable hard drives, gigabit interface converters,small-form-factor pluggable devices, and eas-ily connected cables for both copper and opti-cal connections. A drive shelf allows adding orremoving and replacing SAN drives by simplyplugging or unplugging them.

Fibre Channel roadmapThe Fibre Channel Industry Association (http://

www.fibrechannel.org) works in conjunction withthe T11 standards body to determine market needsand establish a roadmap for Fibre Channel’s con-tinued development. The FCIA’s roadmap coversFibre Channel’s target markets, speeds, and feeds.

The current speeds-and-feeds roadmap calls for1.0625 Gbaud per second, 2.125 Gbaud per secondand 12.75 Gbaud per second for SAN infrastructures,and 4.25 Gbaud per second for intrachassis FibreChannel. Current Fibre Channel implementationsroutinely achieve a performance of 2.125 Gbaud persecond, providing excellent price per bandwidth.

Fibre Channel has already made inroads into themidrange storage market and is making progress inthe entry-level SAN marketplace. As it migrates from

the high-end enterprise-class SAN solutionsto entry-level SAN systems, the technologycontinues to provide superior reliability, per-formance, and scalability.

Path of least resistanceFibre Channel has become the incumbent

protocol for implementing SANs. Currenteconomic conditions and a heightened senseof the need for comprehensive disaster recov-ery and backup plans have led to a conservativeapproach that favors this technology.

The Fibre Channel protocol is relatively simple tolearn. The real learning curve with SANs is under-standing storage, which is protocol independent.This issue is important to the industry in general,especially now that an ever-growing number oftrained, SAN-aware technicians are available tohelp create, install, and maintain SANs.

With years of experience, mature standards, anda comprehensive roadmap, Fibre Channel offers asolid alternative to SAN managers. Designed forhigh-reliability systems and the assurance of data pro-tection and backup, Fibre Channel optimizes SANswhile providing reliable performance and scalability.

Thomas Hammond-Doel is a director of technicalmarketing at Vixel. He received a BS in electricalengineering from the University of Washington.Contact him at Tom.Hammond-Doel@Vixel.com.

FireWire as a Mass-Storage InterfaceEric AndersonApple Computer

B ased on work done largely at Apple Com-puter, in partnership with early adopters suchas Sony and Texas Instruments, developers

completed the IEEE 1394 Standard for a HighPerformance Serial Bus in 1995. Both Apple andthe 1394 Trade Association refer to IEEE 1394 asFireWire (http://www.1394ta.org). Although theterm has no official meaning, like Ethernet andBluetooth it helps customers identify a technologywhose official name consists of an abstract num-ber.

In 1995, digital video camcorders became thefirst major products to incorporate FireWire, fol-lowed by computers in 1997 and mass storageproducts early in 1999. Today, a wide variety ofadditional products use FireWire—including con-sumer products such as cameras, scanners, print-

ers, televisions, game consoles, and many kinds ofstorage devices.

In 2000, the IEEE released the 1394a update.Although it adds no major features, this updateallows significant performance improvement andclarifies many previous ambiguities. Most com-puter and storage products using FireWire nowsupport this updated version.

FireWire’s feature setA 1394 bus can simultaneously support packet

transfer at 100, 200, and 400 Mbps. A single buscan freely mix up to 63 devices supporting any or allof these speeds. The bus transports each packet atthe best possible speed depending on the source anddestination nodes, so adding a digital video cameracapable of only a 100-Mbps rate need not force

Fibre Channeloptimizes SANswhile providing

reliable performanceand scalability.

vice read and write requests by direct memoryaccess to the specified memory address, withoutassistance from the local CPU or any interrupts. Apacket sent to an address outside this physical-memory range in the computer can cause an inter-rupt, which host software interprets. With thisarchitecture, each storage device added to aFireWire bus acts as an additional DMA controllerand can move payload data to or from host mem-ory at its own pace.

FireWire uses the serial bus protocol, version 2,to define this operation. SBP-2 specifies how thehost can write storage commands in its own mem-ory, then signal a storage device to read and exe-cute those commands. SBP-2 provides options forthe host to append additional commands duringexecution. It also lets the device optimally reorderthe execution of commands, assuming the hostallows it.

Once the device has completed a command, itcan inform the host by sending a packet to a hostaddress to cause an interrupt. The host then knowsthat data has been transferred to or from its mem-ory and that the command structure it prepared isavailable for reuse. FireWire can execute the trans-fer of many megabytes with only a single host inter-rupt to signal completion.

Storage products using FireWireFireWire hard disk drives are used as ordinary

add-on storage. Drives powered solely by FireWireare also used as lightweight, high-capacity portablemedia. Several manufacturers offer FireWire CDand DVD drives for use with lightweight notebookcomputers that do not have an internal opticaldrive. FireWire DVD writing drives have recentlyincreased in popularity because few computershave these drives built in. Drives for Zip, magneto-optical, tape, and other media can be ordered withFireWire interfaces.

In addition to providing an easy and quick wayto add, upgrade, and share computer storagedevices, FireWire also has been adopted for high-performance RAID applications. A single FireWireinterface can deliver more than 40 Mbytes per sec-ond for RAID, while PCI cards can add more chan-nels at a low cost. Further, FireWire has been usedfor high-end jukebox-type storage products thatoffer more than a terabyte of capacity.

FireWire is most powerful when it connects mul-tiple devices. These devices may interoperate aspeers to deliver greater capability as a system thaneach device offers by itself. Apple’s Target DiskMode demonstrates this potential: One computer

52 Computer

computers and storage devices to slow downfrom the 400-Mbps rate they commonly sup-port. Packets directed to or from the DV cam-era itself will require the slower rate.

FireWire devices can be connected or dis-connected from the bus at any time. Self-describing and self-configuring, these devicesfree users from having to adjust ID selectorsor assign drivers to devices individually.Electrically, a network of point-to-point ser-

ial links appears as a bus above the physical layer.Like a bus, all packets are visible to all nodes, withthe exception that higher-speed packets cannot besent to or through lower-speed devices.

FireWire provides up to 45 watts of electricalpower for operating devices on the bus. This fea-ture, absent from other common storage buses,ensures that devices with modest power require-ments do not require an extra power connection.For example, Apple’s iPod MP3 player usesFireWire as its only data and power connection.The player can recharge its built-in battery at thesame time it downloads new music from a com-puter.

FireWire uses isochronous transport to providereal-time data delivery, reserving up to 80 percentof the bus bandwidth for one or more isochronouschannels. The hardware then provides guaranteedtransmit opportunities at defined intervals to ensuretimely data transfer. DV cameras, among otherdevices, use the isochronous service, which is par-ticularly efficient because it makes unused reservedbandwidth immediately available for asynchronousdevices, such as storage devices.

A memory model defines FireWire’s asynchro-nous transport. Asynchronous requests indicate adesire to read or write data at a 64-bit address onthe bus. The high 16 bits of this address select anode—and, in the future, a bus—while the remain-ing 48 bits select a memory address within thatnode. Writes can be completed with an immediateacknowledgment that yields a unified transaction.A write that cannot obtain immediate confirmationcan be completed as a split transaction. Althoughreads always use a split transaction, they can sendthe response packet immediately—without arbi-tration—if they can provide the requested data veryquickly.

Memory-mapped I/O and DMAIn FireWire’s open host controller interface speci-

fication, asynchronous packets addressed to thelower portion of a host node’s 48-bit address spacecan directly address host memory. OHCI can ser-

FireWire provides an easy and quick

way to add, upgrade,and share computer

storage devices.

emulates a FireWire disk drive that a second com-puter accesses. This role reversal by the first com-puter demonstrates FireWire’s flexibility, but showsonly a small part of FireWire’s potential to enablefurther innovation.

FireWire is particularly useful for connectingmultiple devices. Using peer-to-peer access, devicescan cooperate to provide more services than theycould alone or when connected to a central com-puter by a different interconnect.

December 2002 53

To date, FireWire has been used only inlimited applications, such as Apple’s TargetDisk Mode. More uses for the power it offersremain to be offered in future products.

Eric Anderson is a manager of FireWire Soft-ware at Apple Computer. Anderson receiveda PhD in computer science from the Univer-sity of California, San Diego. Contact him atewa@apple.com.

Serial ATAMike AlexenkoMaxtor

Serial ATA’s point-to-point

topology provides

a direct pathbetween the

disk drive and the host.

A next-generation technology, the Serial Ad-vanced Technology Attachment interface con-nects disk drives to the PC platform. SATA has

its roots in the parallel ATA interface, which has beenthe most popular disk drive interface in terms ofdevices shipped. In 2001, more than 172 million diskdrives shipped with the parallel ATA interface.Adding to this figure the volume of optical andremovable storage devices and host interfaces withATA suggests that an immense number of ATA portsare in use.

Parallel ATA’s popularity stems in part from itsrelatively low cost and simplicity. However, it is astorage-centric bus with limited scope that addressesonly two storage devices inside the box on an 18-inch cable, with a simple protocol and a single host.

Two factors are driving the transition from paral-lel ATA to SATA: signaling voltages and the numberof signals the interface requires. Parallel ATA requires33 signals on a host controller using an 80-conduc-tor cable, as well as a 5-volt tolerance. Both of theserequirements limit the future potential of parallelATA because semiconductor technology advanceshave lowered operating voltages and die sizes.

Faced with the continued need to scale interfacedata rates to match disk drive data rates and therequirements of parallel ATA’s physical plant, sev-eral companies formed a group to define the long-term, software-compatible replacement for parallelATA. The Serial ATA Working Group introducedthe first SATA specification, Serial ATA 1.0, in 2001(http://www.serialata.org).

SATA’s benefitsThe Serial ATA Working Group seeks to develop

a replacement for parallel ATA that maintains thecharacteristics that made parallel ATA popular: lowcost, simplicity, and limited scope. SATA technol-

ogy’s acceptance and success in the desktop andmobile computing markets requires that it emulatethese characteristics. Thus, key SATA featuresinclude the following:

• a 10-year technology roadmap that starts withdata transfer rates of 150 Mbps, scales to 300Mbps and, eventually, 600 Mbps;

• 100 percent device-driver compatibility be-tween SATA and parallel ATA;

• 250-mVolt signaling levels;• improved wireability, enabled by a seven-wire

data interface with four data signals and apoint-to-point star topology;

• blindmate connector with data and powerbays, ideal for both cabled and backplane envi-ronments;

• hot-plug support;• cable lengths that extend to 1 meter; and• improved reliability with 32-bit cyclic redun-

dancy checking for all transfers and improvedsignal integrity.

While addressing the needs of the desktop andmobile computing platforms, some enhancementsthat SATA defines—such as blindmate connector,point-to-point topology, and hot-plug support—make it attractive for other applications as well.

SATA applicationsDevice cost and cost per gigabyte drive the use

of ATA-class disk drives in network storage appli-cations. System integrators building cost-sensitiveentry servers or devices that package the highestpossible volumetric density have been turning toATA-class disk drives. This trend is especiallyprevalent when performance, measured in I/Os persecond, takes a back seat to cost and capacity.

54 Computer

Table 2. Potential Serial ATA roadmap.

Year Event Data transfer rate

2001 First-generation specification 150 Mbps2002 First SATA products2003 Integrated SATA solutions2004 Second-generation specification 300 Mbps2005-2006 Market continues to build and SATA feature

set grows, enabling new applications2007 Third-generation specification 600 Mbps

SATA’s point-to-point topology provides a directpath between the disk drive and the host, which letsengineers easily aggregate many disk drives in a sin-gle system. This topology offers additional benefitsincluding

• dedicated bandwidth between a device and thehost;

• the ability to more easily isolate device failures;and

• performance scalability based on the devicedata rate, not on speed-matching betweendevices.

iSCSI ProtocolTom ClarkNishan Systems

As SATA matures, it will continue to replace par-allel ATA disk drives, including applications out-side traditional PC computing, such as personalvideo recorders, electronic gaming, and some net-work storage applications.

Making SATA a realityThe SATA 1.0 specification has already acquired

more than 100 contributors, promoters, andadopters. Since 2001, many companies have pro-duced SATA device samples and technology andproduct demonstrations. SATA is well on its way tobecoming the most popular disk interface, replac-ing parallel ATA in a variety of applications. Table2 shows SATA’s anticipated future developmentpath.

Work has already begun on Serial ATA II,whose goals include definition of 3-Gbps signal-ing (enabling 300-Mbps data-transfer rates),enhanced command queuing, and enhanced con-nectivity.

Mike Alexenko is a senior director of I/O strategyat Maxtor. Alexenko received a BS in computer sci-ence from Wichita State University. Contact himat Mike_Alexenko@maxtor.com.

T he Internet SCSI protocol has been developedto transport storage commands, status, anddata over mainstream IP networks. Like Fibre

Channel, iSCSI encapsulates SCSI commanddescriptor blocks in a serial stream. Serializingblock I/O—storage area networking’s enablingtechnology—allows the flexible deployment ofshared storage resources such as servers, diskarrays, and tape subsystems. While Fibre Channeluses a dedicated gigabit network infrastructure sep-arated from the corporate LAN, iSCSI uses main-stream Gigabit Ethernet switching that can betightly integrated into the corporate LAN.

As Figure 1 shows, iSCSI carries SCSI data overTCP/IP. Routing packets across the networkrequires an IP layer, while the TCP layer ensuresend-to-end session control and lost-packet recov-ery. With the widespread deployment of gigabit-switched-optical networks, bandwidth and con-gestion cause less concern.

Given minimal packet loss through the network,the upper-layer TCP recovery mechanism is rarely

invoked. The TCP layer serves primarily as an insur-ance policy against infrequent network disruptions,its packet retransmission ensuring that while indi-vidual packets may be lost, data never will be.

The iSCSI protocol shares many features withFibre Channel. Both use an underlying high-speedserial transport to carry SCSI read and write com-mands to storage targets. They both have a vari-able network address identity and a fixed uniqueworldwide name address. They both also rely onflow-control mechanisms to maintain stable con-versations between pairs of communicating devicesacross the network.

An iSCSI SAN differs from a Fibre Channel SANprimarily in the plumbing that supports SCSIexchanges. Fibre Channel provides a layer 2 archi-tecture, analogous to bridged LANs before theintroduction of TCP/IP. iSCSI relies on a layer 3architecture that includes network routing. OnceSCSI commands, status, and data are put in an IPformat, storage data can use conventional equip-ment to traverse any common IP network infra-

December 2002 55

SCSISCSI bus protocol

Serial SCSI-3 transport

TCP/IP network (SAN,LAN, MAN, WAN)

Internet protocol

iFCP or iSCSI

SCSI command set

Figure 1. Internet SCSI protocol. iSCSI uses an IP layer to route SCSI data packets across the network, then uses theTCP layer to ensure end-to-end session control and lost-packet recovery.

iSCSIhost

iSNSserver

iSCSItarget

iSCSItarget

iSCSIhost

IP network

Figure 2. iSCSI storage area network. This pristine SANincludes servers and storage systems with native IPinterfaces to the network.

structure—including Gigabit Ethernet, ATM,Sonet, or Frame Relay.

Figure 2 shows an iSCSI SAN that includesservers and storage systems with native IP inter-faces to the network. The network itself can becomposed of Gigabit Ethernet switches, IP routers,wide area links, and switched-optical network seg-ments. However efficient the IP network may be,upper-layer storage applications can be sensitive toobjective influences such as speed-of-light latency.

Streaming applications such as tape backup cantypically tolerate more latency over distance thansynchronous disk-mirroring applications thatexpect acknowledgment for every transaction. Aswith Fibre Channel SANs, basic network designshould accommodate the iSCSI transactions’ band-width requirements.

By integrating storage data into mainstream datacommunications infrastructures, iSCSI makesshared storage access ubiquitous. This creates newmarkets for SAN solutions and new engineeringinitiatives for iSCSI product development and SANmanagement.

Tom Clark is the director of technical marketing atNishan Systems. Clark received a BA in historyfrom the University of Kansas. Contact him attclark@NishanSystems.com.

T aken collectively, these new storage technolo-gies offer customers a broad selection of solu-tions, from high-end, high-performance

storage networks for enterprise applications tomore flexible deployment of storage for worksta-tion environments. Hybrid storage products mayintegrate different storage technologies to achievegreater economies and storage densities. SerialATA, for example, can provide backend storage for

RAID controllers whose network interfaces areFibre Channel or iSCSI.

The convergence of storage architectures posesnew challenges to developers who wish to optimizeperformance and interoperability without incurringadditional design overhead. As compensation, themarket for innovative storage solutions continuesto expand and offers new opportunities for faster,higher-capacity, and lower-cost storage devices. �

Michael T. LoBue is president and chief executiveofficer of LoBue & Majdalany Management Group,an association management company. LoBue’s firmmanages enterprise storage and computing tradeassociations, including the SCSI Trade Associationand Fibre Channel Industry Association. LoBuereceived an MS in management and public policyfrom Carnegie Mellon University. Contact him atLoBue@LM-Mgmt.com.