Description of System Area Networks


A System Area Network (SAN) is a new class of high-performance communication interconnector that has recently become available for workstation or PC clusters. Windows 2000 Datacenter Server supports SANs with certain hardware.

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System Area Networks deliver very high bandwidth (more than 1 GB/second) with very low latency. SANs are switched, with a typical hub supporting 4-8 nodes. You can build larger networks from cascading hubs. Cable length limitations can vary from a few meters to a few kilometers.

A SAN is typically used to interconnect nodes within a distributed computer system, such as a cluster. These systems are members of a common administrative domain and are usually in close physical proximity. A SAN is assumed to be physically secure.

These new interconnections differ from existing high-performance media, such as Gigabit Ethernet and ATM, in several ways. SAN network adapters implement reliable transport service, akin to TCP or SPX, directly in hardware. The network adapter controller exposes individual transport endpoints and demultiplexes incoming packets accordingly. Each endpoint is usually represented by a set of memory-based queues and registers that are shared by the host processor and the network adapter controller. Many SAN network adapter controllers permit these endpoint resources to be mapped directly into the address space of a user-mode process. Program processes can post messaging requests directly to the hardware, with no system calls and no intermediate data copying. Fully reliable SAN network adapter controllers, with direct user-mode access, can deliver extremely good message-passing performance.

SANs have very low error rates. SANs are often made highly available by using redundant interconnect fabrics. SAN performance more closely resembles that of a memory subsystem than a traditional network, such as a LAN.

SANs provide bulk data transfer through a Remote Direct Memory Access (RDMA) mechanism. The initiator specifies a buffer on the local system and a buffer on the remote system. Data is then transferred directly between the two locations by the network adapters without host CPU involvement at either end. Both read (get) and write (put) transfers can be supported. The remote address must be prearranged through a message exchange. Some designs also permit the transfer of a small amount of immediate data on RDMA operations. This data is delivered to the remote process when the RDMA transfer has finished and is used in lieu of a subsequent message.

A new specification has been implemented in Winsock called "Windows Sockets Direct Path" (WSDP) for SANs. This new technology enables Windows Sockets programs that use TCP/IP to obtain the performance benefits of SANs without program modifications.

A new layer called "switch" is inserted between Windows Sockets Interface and Windows Sockets Service Providers (Microsoft Base Winsock Provider and SAN Winsock Provider). This component emulates TCP/IP semantics over native SAN service providers. It is referred to as the "switch" because it transparently switches between the standard TCP/IP Windows Sockets provider and SAN providers on a per-socket basis.

Compaq and Giganet are among the third-party vendors with products supporting this technology. For more information, refer to these Web sites: For more information about the Winsock Direct Specification and Winsock Direct Guidance for Vendors, visit the following Microsoft Web site: NOTE: Currently Emulex has bought giganet cards. For more information, visit the following Emulex Web site: For additional information, click the article number below to view the article in the Microsoft Knowledge Base:
264135 Differences with System Area Networks and Storage Area Networks
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Artikelnummer: 260176 – Letzte Überarbeitung: 04.12.2008 – Revision: 1