IP storage switches complement Fibre Channel SANs

Storage networking hardware includes three basic components: host adapters, storage targets and switch interconnects. In traditional SANs, these basic building blocks have been based on Fibre Channel technology, with Fibre Channel interfaces for host bus adapters and storage arrays and Fibre Channel fabric switches providing network connectivity.

The complexion of SANs is now evolving, however, to include heterogeneous combinations of Fibre Channel and IP technologies. The introduction of IP storage protocols such as iSCSI (Internet SCSI), iFCP (Internet Fibre Channel Protocol) and FCIP (Fibre Channel over IP) is enabling the convergence of traditional Fibre Channel and mainstream IP networking, as well as the construction of storage networks in which one or more of the host, target or switch components are based on TCP/IP.

The fusion of these technologies is embodied by IP storage switches, which are designed to translate between IP and Fibre Channel environments.

IP storage switch products currently have two primary applications. In the first application, an IP storage switch may be used to connect Fibre Channel assets over an IP network. FCIP, for example, may be used to connect Fibre Channel switches over IP-based wide-area networks (WANs), typically for disaster recovery applications. Likewise, the iFCP protocol is used to connect either Fibre Channel switches or Fibre Channel end devices over IP.

In both cases, the host systems and storage targets remain Fibre Channel. The second primary application for IP storage switches is connecting IP-based SAN devices to Fibre Channel devices. This typically involves connecting iSCSI hosts to Fibre Channel storage targets. In this case, two of the three basic SAN components may be IP, with Fibre Channel restricted to the storage array or tape subsystem.

Not all IP storage switch products support both IP storage applications concurrently. Some vendors may only provide SAN extensions by connecting existing SAN islands over IP networks using FCIP. Some may only provide iSCSI host-to-Fibre Channel target connectivity. At the high end of the product offerings, multi-protocol IP storage switches enable any-to-any connectivity to accommodate internetworking of Fibre Channel SANs, Fibre Channel end devices and iSCSI host or storage targets.

SAN extension

SAN extension enables use of IP wide-area services to connect remote SANs over distance. This has significant advantages for customers that want to implement remote storage applications since IP WAN services are both available and affordable. By contrast, a native Fibre Channel extension requires dedicated fiber (typically using dense wave division multiplexing or DWDM) and cannot span beyond metropolitan distances.

In a SAN extension scenario, two geographically dispersed Fibre Channel SANs are connected over an IP network by IP storage gateways. At each site, the local IP storage gateway is attached by expansion port (E_Port) to one of the local Fibre Channel switches. Fibre Channel frames sent from one site to another are wrapped in IP datagrams for transport across the IP network. At the receiving end, the IP datagram is removed and native Fibre Channel frames are issued to the destination Fibre Channel switch.

This IP tunneling scheme preserves Fibre Channel SANs at both ends while only making opportunistic use of the IP network to accommodate the distance between them. When FCIP is used for SAN extension, the entire IP storage gateway and IP network span is transparent to the Fibre Channel switches. The two connected fabric switches therefore engage in normal Fibre Channel fabric building and create a single link layer Fibre Channel fabric over the stretched E_Port connection.

The drawback of this solution is that a disruption on one local site may cause a disruptive fabric reconfiguration at both sites. IP tunneling using FCIP solves the distance problem but may inject instability in the extended SAN due to local occurrences or disruption in the IP cloud.

SAN internetworking

SAN internetworking addresses the stretched E_Port issue by enabling autonomy for each local SAN while providing connectivity for storage transactions across the IP network. IP storage switches that support SAN internetworking may connect to local storage devices via E_Port to an existing Fibre Channel switch, or by direct connection if the iFCP protocol is used. Unlike FCIP tunneling, however, the E_Port connection is terminated at the IP storage switch.

In this scenario, switch-to-switch protocols and fabric building are not extended across the IP network. Instead of creating a flat link layer SAN stretched over IP, the SAN internetworking solution preserves the autonomy of each local SAN. A disruption at one site is therefore isolated, and disruptive fabric reconfigurations are avoided. SAN internetworking also enables multi-point connectivity for complex storage networks, allowing multiple geographically dispersed sites to be connected for applications such as consolidated tape backup or multi-point disaster recovery.

IP storage in the data center

IP storage switches may also be used for data center storage applications. Carlson Companies, for example, has implemented a SAN that, while using Fibre Channel adapters in servers and Fibre Channel storage arrays, substitutes IP storage switches and Gigabit Ethernet core switches in place of traditional Fibre Channel directors.

This enables use of familiar IP and Ethernet to build the SAN and facilitates integration of IP-based remote backup from Carlson's dispersed locations. For this application, iFCP provides direct connectivity for storage devices and servers, without requiring Fibre Channel fabric switches.

iSCSI attachment

iSCSI was designed as a pristine IP storage architecture to provide connection for both servers and storage arrays in an IP network. Although iSCSI device drivers and iSCSI adapter cards for servers are now available, the majority of enterprise-class storage targets are still based on Fibre Channel. IP storage switches are therefore required to bring iSCSI hosts to Fibre Channel targets. Product differentiation is primarily based on performance and the number of devices supported.

iSCSI-to-Fibre Channel protocol conversion requires store-and forward-buffering, frame parsing, (AKA, frame cracking), address translation and conversion of Fibre Channel Protocol (FCP) commands, status and data to iSCSI commands, status and data. Ideally, this protocol conversion should be executed at wire speed, although not all products support full Gigabit performance. The combination of IP storage switches and iSCSI host attachment dramatically reduces the Fibre Channel content of a SAN and enables customers to leverage more of their mainstream IP assets and support infrastructure.

Summary

IP storage switches complement, and in some cases amplify, the benefits of Fibre Channel SANs by providing more economical solutions for storage over distance, SAN internetworking with fault isolation, and integration of both IP-based and Fibre Channel-based storage resources. This gives customers more options for implementing shared storage solutions by combining the best of traditional SANs with the best of mainstream IP networking.

Tom Clark is director of technical marketing at Nishan Systems Inc. He can be reached at tclark@nishansystems.com

Copyright © 2003 IDG Communications, Inc.

  
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