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Introduction to SAN technology

SANs overcome the problems of traditional network storage by moving storage resources off the common user network and reorganizing those storage components into an independent, high-performance network. SANs also support storage features such as disk mirroring, data backup, data archiving/retrieval and data migration.

All networks include some form of storage. Traditional network storage was located inside, or directly connected to, individual file servers that were scattered across workgroups throughout an organization. This resulted in a cumbersome, complicated, multiple-server environment that was impossible to organize or secure.

A storage area network (SAN) overcomes these problems by moving storage resources off the common user network and reorganizing those storage components into an independent, high-performance network. Storage performance is enhanced using a fast interface (e.g., 2 Gbps or 4 Gbps Fibre Channel) (FC) that connects storage servers and storage devices through switches and hubs to form a fabric that supports both redundancy and high availability. SAN technology also supports storage features such as disk mirroring, data backup/restoration, data archiving/retrieval and data migration.

SAN components and architecture

Most SANs are assembled using three components: cabling, host bus adapters (HBA) and switches. Cabling is the physical medium used to interconnect every SAN device. FC SANs can use copper or optical fiber cabling -- the choice depends on the speed and distance requirements of the SAN. Slower or shorter distance connections can be made through copper cables; faster or longer distance connections are achieved through optical cables.

Optical fiber cables can be single mode or multimode. Single-mode (or monomode) fiber is designed to carry only one light signal over long distances, while multimode fiber can carry multiple simultaneous light signals over short distances. Optical fiber uses different kinds of connectors, so it's important to select connectors that are compatible with other components of the fabric.

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Each server or storage device in a SAN fabric requires an HBA. The HBA can be either an expansion card that fits into a compatible expansion slot in a server, or a chip integrated directly into the server or storage device. An HBA offloads SAN traffic overhead from the local processor, improving server performance. Cabling connects the HBA's port to a corresponding port on a switch.

A switch handles and directs traffic between network devices. The switch accepts traffic, then relays the traffic to the port where the intended destination device is attached. In a SAN, each storage server and storage device connects to a switch port. The switch then relays traffic to and from specific devices across the SAN. This series of switched interconnections form the SAN "fabric," which can easily be scaled or changed. An intelligent switch serves the same basic functions but incorporates high-level SAN features such as storage virtualization, quality of service (QoS), remote mirroring, data sharing, protocol conversion and security.

SAN connectivity and protocols

SANs are also defined by their interconnection scheme, usually either FC or iSCSI. FC -- the most popular approach for enterprise SANs -- supports low-latency communication between servers and storage devices at 2 Gbps (although 4 Gbps implementations are now common and 10 Gbps implementations are coming). Traditional FC uses optical fiber cables to interconnect devices and is still employed over long distances. Today, short-distance FC implementations can be achieved with coaxial and twisted-pair copper cables. FC can operate directly between two devices (point-to-point), or network multiple storage devices through a switch or arbitrated loop. FC technology is compatible with SCSI and IP protocols.

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Internet SCSI (or iSCSI) is an emerging Internet Engineering Task Force standard that allows SCSI commands to support data storage and retrieval over Ethernet networks, which include LANs, WANs and the Internet. By leveraging the broad acceptance of IP networks, iSCSI technology is expected to strengthen the SAN market and has already found acceptance in small and medium-sized companies for basic SAN deployments.

Since Ethernet networks generally work up to 1 Gbps, iSCSI isn't as fast as FC, which starts at 2 Gbps. However, iSCSI performance is acceptable for many applications. iSCSI is less expensive than FC, and Ethernet is well-understood by any IT professional. In addition, 10 Gbps Ethernet is on the horizon and could also threaten FC's established position as the SAN networking technology of choice.

SAN management

Creating a SAN is more involved than simply cabling servers and storage systems together. Storage resources must be configured, allocated, tested and maintained as new devices are added and enterprise storage requirements change. Management is a vital part of SAN operation, so it's important to select tools that can minimize the time and effort needed to keep a SAN running.

Storage resource management (SRM) applications monitor and manage physical and logical SAN resources. Physical resources include storage arrays, RAID systems, tape libraries and FC switches; logical storage features involve file systems and application-oriented storage elements (e.g., Oracle database files).

It's usually best to select one tool that can provide centralized management of the entire storage infrastructure through a single console. A centralized SRM tool should ideally be able to detect storage resources, evaluate their capacity and configuration, measure their performance and even help plan for future expansion. The SRM tool should also be able to effect changes to the configuration and support consistent policies across the storage technologies being managed. SAN management tools are available from EMC Corp., Symantec Corp. (Veritas), Brocade, Hewlett-Packard Co., IBM, Sun Microsystems Inc. and CA.

Selecting a SAN management/SRM tool can be extremely challenging, because each tool accomplishes its suite of tasks in a unique way. Consequently, a good management tool should offer heterogeneous support, being able to accurately detect, discover and visualize a SAN across a variety of network equipment, storage systems and operating systems. The tool should provide meaningful monitoring and reporting features, including performance measurement, and that data should provide practical information that can help an administrator identify and resolve problems within the SAN.


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