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Monday, Dec 23, 2002

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From LAN to SAN for data storage

Arvind Padmanabhan

SAN is a specialised network that enables fast, reliable access among servers and external or independent storage resources. Just as a LAN can be used to connect clients to servers, a SAN can be used to connect servers to storage, servers to one another and storage to storage.

IN THE post-September world, business continuance — the ability to get your network up and running after a disaster with minimal impact on business — has become crucial and this has spawned storage-area network, or SAN.

SAN is a dedicated, centrally-managed, secure information infrastructure, which enables any-to-any interconnection of storage and systems. It offers simplified storage management, scalability, flexibility, and improved data access, movement and backup. SAN is a high-speed network, comparable to a LAN (Local Area Network), that allows the establishment of direct connections between storage devices and processors (servers) centralised to the extent supported by the distance of fibre channel. SAN is a specialised network that enables fast, reliable access among servers and external or independent storage resources. In SAN, a storage device is not the exclusive property of any one server. Rather, storage devices are shared among all networked servers as peer resources; this includes data and resource sharing. Just as a LAN can be used to connect clients to servers, a SAN can be used to connect servers to storage, servers to one another and storage to storage.

SAN does not need to be a physically separate network, either. It can be a dedicated sub-network, carrying only the business-critical I/O traffic between servers and storage devices. SAN, for example, would not carry general-purpose traffic such as e-mail or other end-user applications. This type of net avoids the unacceptable trade-offs inherent in a single network for all applications, such as the need for dedicated storage devices for each server and burdening a LAN with storage and archival activity. According to the research company IDC, SAN will account for 70 per cent of all the network storage by 2004. SAN evolved from the concept of taking storage devices, and therefore storage traffic, off the LAN and creating a separate back-end network designed specifically for data.

SAN is very useful and good for a transactional/highly available database, for large blocks of data, Data Warehousing/ERP applications, broadcasting and printing prepress environments. It allows storage devices to exist on their own separate network and communicate over a very fast media. Users can gain access to these storage devices through server systems which are connected to both LAN and the SAN. SAN addresses bandwidth bottleneck, clustering, provides more modularity, manageability, ease of integration, high availability, increased fault tolerance, centralised storage management, server less backup.

The starting point for implementing SAN is determining the criticality of the data storage, how fast is fast enough, and to what degree will it need to change in the future. This means getting optimal performance, maximising utilisation of the resources, providing open interfaces to a variety of application needs, and having the widest range in scaling performance, availability, and reliability.

From a client network perspective, the SAN environment complements the ongoing advancements in LAN and WAN technologies by extending the benefits of improved performance and capabilities all the way from the client and backbone through to servers and storage. The advantages of SAN has led to an increasing amount of popularity; as they are quite simply better suited to address the data storage needs in today's data intensive network environment. SAN promises to revolutionise the network-centric, data intensive computing era through a new innovative market space. SAN adds bandwidth for specific functions without placing a load on the primary network, it also enables higher performance solutions such as data warehousing.

SAN is most commonly implemented using a technology called Fibre Channel deployed by American National Standards Institute (ANSI), supports data rates over 2 Gbps. Fibre Channel was originally spelelt "Fiber" Channel. When the specification expanded to include copper and cat 5 cables, it changed to "Fibre" to distinguish it from Fiber Optics. Since, Fibre Channel is a switched technology; it is able to provide a dedicated path between the devices in the fabric so that they utilise the entire bandwidth for the duration of communication.

The storage devices are connected to Fibre Channel switch using either multimode or single mode fibre optic cable. Multimode is used for short distances (up to 2 km) and single mode for longer distances. With its support for both Internet Protocol (IP) and Small Computer System Interface (SCSI), Fibre Channel is the first truly open networked storage technology. As the world goes increasingly wireless, it could also redefine the concept of "data storage on the go" — with networks that let users store, retrieve and share just about anything from personal music collections to important client information and lifesaving data in emergencies.

A report by Datacomm Research contends that wireless connectivity will boost growth in the burgeoning data storage market, making possible new applications for database access and content sharing.


These three IP storage networking transports are significantly different, but all provide a common function: Transporting block-level storage over an IP network. All three transports enable end users to:

Leverage existing storage devices (SCSI and Fibre Channel) and networking infrastructures (Gigabit Ethernet);

Maximise storage resources to be available to more applications;

Extend the geographical limitations of SAN access;

Use existing storage applications (backup, disaster recovery, and mirroring) without modification; and

Manage IP-based storage networks with existing tools and IT expertise.

The Internet Small Computer Systems Interface (iSCSI) protocol defines the rules and processes to transmit and receive block storage applications over TCP/IP (Transport Control Protocol / Internet Protocol) networks by encapsulating SCSI commands into TCP and transporting them over the network via IP.

Fibre Channel over TCP/IP (FCIP) provides a mechanism to `tunnel' Fibre Channel over IP-based networks. This enables the interconnection of Fibre Channel SANs, with TCP/IP used as the underlying wide-area transport to provide congestion control and in-order delivery of data.

The Internet Fibre Channel Protocol (iFCP) supports Fibre Channel Layer 4 FCP over TCP/IP. It is a gateway-to-gateway protocol where TCP/IP switching and routing components complement and enhance, or replace, the Fibre Channel fabric.

Even with all the intrinsic benefits, SAN is not the cure-all for all storage management headaches. While many of the problems associated with server attached storage are solved by SAN; they may raise a whole new spectrum of issues. Like any new technology, solving one set of problems tends to highlight additional issues that need to be understood and acted upon.

Achieving the ultimate benefits of SAN implementation requires architecting an underlying virtualisation scheme with enterprise-class features, along with a flexible approach that scales with future needs.

SAN is in the domain of big business and expensive now. But prices will fall, and SAN will find its way into organisations. It is refreshing when a single technology yields both unmatched performance and exceptional total cost of ownership benefits.

The SAN platform supports gigabit speed storage access, server networking and server storage clustering on one extensible platform.

(The author is a software professional with Wipro.)

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