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NETWORK ATTACHED STORAGE ARCHITECTURE

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COMMUNICATIONS OF THE ACM November 2000/Vol. 43, No. 11 37SAN with Fibre Channel network hardware that hasa greater effect on a user’s purchasing decisions. Thisarticle is about how emerging technology may blurthe network-centric distinction between NAS andSAN. For example, the decreasing specialization ofSAN protocols promises SAN-like devices on Ether-net network hardware. Alternatively, the increasingspecialization of NAS systems may embed much ofthe file system into storage devices. For users, it isincreasingly worthwhile to investigate networkedstorage core and emerging technologies.Today, bits storedonline on magneticdisks are so inexpen-sive that users are find-ing new, previouslyunaffordable, uses forstorage. At Dataquest’sStorage2000 confer-ence last June inOrlando, Fla., IBMreported that online disk storage is now signifi-cantly cheaper than paper or film, the dominanttraditional information storage media. Not surpris-ingly, users are adding storage capacity at about100% per year. Moreover, the rapid growth of e-commerce, with its huge global customer baseand easy-to-use, online transactions, has introducednew market requirements, including bursty, unpre-dictable spurts in capacity, that demand vendorsminimize the time from a user’s order to installationof new storage.In our increasingly Internet-dependent business and computing environment, network storage is the computer.NETWORK ATTACHEDSTORAGE ARCHITECTURETHE GROWING MARKET FOR NETWORKED STORAGE IS A RESULT OF THE EXPLODING DEMANDFOR STORAGE CAPACITY IN OUR INCREASINGLYINTERNET-DEPENDENT WORLD AND ITS TIGHTLABOR MARKET. STORAGE AREA NETWORKS(SAN) AND NETWORK ATTACHED STORAGE(NAS) ARE TWO PROVEN APPROACHES TO NETWORKING STORAGE. TECHNICALLY, INCLUDINGA FILE SYSTEM IN A STORAGE SUBSYSTEM DIF-FERENTIATES NAS, WHICH HAS ONE, FROMSAN, WHICH DOESN’T. IN PRACTICE, HOW-EVER, IT IS OFTEN NAS’S CLOSE ASSOCIATIONWITHETHERNET NETWORK HARDWARE ANDGarth A. Gibson and Rodney Van MeterATOMIC-FORCE MICROSCOPE SCAN OF A RECORDED BIT ON A DISK'S PHASE CHANGEMATERIAL COATING, SIROS TECHNOLOGIES, SAN JOSE, CA38 November 2000/Vol. 43, No. 11 COMMUNICATIONS OF THE ACMThe rapid increase in theneed for online capacityfuels other business tech-nology trends. First, capitalinvestment in storage isnow more than 50% of allcapital investments in cor-porate data centers. Indus-try analysts predict thispercentage will reach 75%by 2003. Second, personnelcosts for storage manage-ment (for, say, tuning per-formance and maintainingbackups) now dominatecapital costs over the equip-ment’s useful lifetime. Ven-dors estimate this recurringcost to be as much as $300per gigabyte per year; thatis, each year’s recurring costis comparable to, and oftenexceeds, its one-time capi-tal-cost counterpart. Coupled with the shortage ofinformation technology professionals and the bursty,unpredictable capacity needs of e-commerce, it istherefore not surprising that a new market hasemerged for data-center outsourcing for both com-plete applications (application service providers) andstorage systems only (storage service providers).Third, the increasing cost of storage management,coupled with the continuing decline in the cost ofstorage capacity, has prompted analysts to predict thatby 2005 the primary medium for storage backup willbe online hard disks. Various system properties are improved by separat-ing storage from application servers and clientmachines and locating it on the other side of a scal-able networking infrastructure (see Figure 1). Net-worked storage reduces wasted capacity, the time todeploy new storage, and backup inconveniences; italso simplifies storage management, increases dataavailability, and enables the sharing of data amongclients. It reduces wasted capacity by pooling devices andconsolidating unused capacity formerly spread overmany directly attached storage servers. It reduces thetime needed to deploy new storage, because clientsoftware is designed to tolerate dynamic changes innetwork resources but not the changing of local stor-age configurations while the client is operating. Databackup traditionally occupies application data serversand hosts for much of the night, a significant incon-venience for global and always-open organizations.With networked storage, backup can be made lessinconvenient, because data can be transferred tooffline tape storage when the devices are not busy—day or night—without application-server involve-ment. Networked storage also simplifies storagemanagement by centralizing storage under a consoli-dated manager interface that is increasingly Web-based, storage-specific, and easy to use. Inherent availability, at least in systems in which allcomponents are provided by the same or cooperatingvendors, is improved, because all hardware and soft-ware in a networked storage system is specificallydeveloped and tested to run together. Traditionalservers are more likely to contain unrelated hardwareand software added and configured by users. Suchuser additions can be destabilizing, because all com-ponents are unlikely to have been sufficiently testedtogether, resulting in more frequent crashes. Finally,the sharing of data among clients is improved,because all network clients can access the same net-worked storage.The principal disadvantages of networked storageare due to the increased complexity of systems spreadacross a network. More machines have to functioncorrectly to get work done; network protocol process-ing is more expensive than local hardware deviceaccess; and data integrity and privacy are more vulner-able to malicious users of the network. While networkstorage technology must address these disadvantages,all distributed and Internet applications share them;hopefully, network storage technology will share theirFigure 1. Networked storage versus direct-attached storage.NetworkDirect-attached storageNetworked storageNetworked storageclientssolutions, as well as their problems.Most networked storage systems fall into one oftwo technology families: NAS systems (such as theF700 file-server systems from Network Appliance ofSunnyvale, Calif.) typically accessed via Ethernet net-works; and SAN systems (such as the Symmetrix diskarray from EMC of Hopkinton, Mass.) typicallyaccessed via Fibre Channel networks [1]. Both NASand SAN storage architectures provide consolidation,rapid deployment, central management, more conve-nient backup, high availability, and, to varyingdegrees, data sharing. It is therefore


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