Differences Between a System Area Network and a Storage Area Network

Overview:

Windows 2000 includes two technologies that use the same acronym, System Area Network (SAN) and Storage Area Network (SAN). This study note provides a description of these two technologies, including their differences.

Storage Area Network:

A Storage Area Network connects multiple servers and storage devices on a single network. This network typically uses Fibre Channel connections and block protocols (SCSI); future implementations may also use Ethernet or other interconnects. SANs also allow sharing the storage infrastructure, without implying data sharing. This allows higher utilization of storage devices and easier reconfiguration than is possible with direct attached storage. Although sharing of storage devices is possible, it is generally not advisable unless the servers connected to the shared devices are running MS Cluster Server.

Precautions must be taken to prevent unintentional access to the storage devices. For switched fabrics, this would involve setting up zones. In any case, devices may implement LUN masking capabilities prevent data corruption caused by unintended access to the storage. 

System Area Network:

System Area Networks, in conjunction with the Windows Sockets Direct Path (WSD) for System Area Networks feature in Windows 2000 Datacenter, are based on a completely different concept than a Storage Area Network. Windows 2000 Datacenter Server-based servers are connected to each other by using a reliable, very-low latency, high speed 1 Giga Bits Per Second (Gbps) + Fiber Channel connection that uses special System Area Network adapters (32/64-bit PCI network adapters). Any Winsock-based program that uses standard Winsock API calls can use System Area Network technology to communicate by using a direct Windows socket from one Windows 2000 Datacenter Server-based server to another.

This capability eliminates the overhead of TCP/IP and provides a reliable, connection-oriented transfer between two endpoints. The technology is based partly on the Virtual Interface Architecture. The Virtual Interface Architecture also provides for additional features such as Remote Direct Memory Access (RDMA). RDMA allows a Windows 2000 Datacenter Server-based server to transfer data from the local computer's physical RAM directly into physical RAM of a remote Windows 2000 Datacenter Server-based server without requiring intervention by the CPU of either computer.

The TCP-based communication that Winsock Direct provides is non-routable. However, fiber channel switches can be used to connect many servers together up to distances of a few kilometers. A typical implementation might involve an eCommerce site where "front-end" Microsoft Windows 2000 Datacenter Server-based servers that are running Microsoft IIS 5.0 are clustered into a Web farm by using Network Load Balancing (NLB) and are connected to backend Microsoft SQL servers that are hosted on Windows 2000 Datacenter-based servers running Microsoft Clustering Service. This "backend"' connection is made by using a System Area Network switch. This would provide very high-speed, low latency access to information that is stored in single or multiple SQL databases.