7.5 IP Storage Gateways

An IP storage gateway may convert Fibre Channel to iSCSI or Fibre Channel to iFCP, or both, depending on product design. An iSCSI-to-Fibre Channel storage switch is typically used to bring iSCSI-enabled hosts to Fibre Channel-attached targets. IP storage gateway products may be designed around fixed-port configurations, with ports or port blades dedicated to either Fibre Channel or Gigabit Ethernet. Alternatively, multiservice port designs allow any port to be used for either Fibre Channel or Gigabit Ethernet attachment. The latter provides more flexibility but must accommodate both Fibre Channel and Gigabit Ethernet signaling as well as protocols on a port-by-port basis.

The early market product in this case was the Cisco SN 5420, which provided a single Gigabit Ethernet port and a single Fibre Channel port with a substantial price tag. The second-generation version of this product is the SN 5428, which improves the port count to provide two ports of Gigabit Ethernet and eight ports of Fibre Channel at a more reasonable price. The underwhelming throughput of the original SN 5420 contributed to the downplay of iSCSI functionality. By virtue of integrating an eight-port Fibre Channel switch-on-a-chip, the SN 5428 offers improved performance.

iSCSI-to-Fibre Channel protocol conversion requires store-and-forward buffering, frame parsing (also called frame cracking), address translation, and the conversion of 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. As shown in Figure 7-5, a fixed-port configuration offers some number of dedicated Gigabit Ethernet ports and typically 8 or 16 Fibre Channel ports. Engineering this type of product is less difficult than designing multiservice ports, but the former is limited by the off-the-shelf components used to create it. Given that the primary function of an iSCSI-to-Fibre Channel gateway is to provide iSCSI host access to Fibre Channel targets, the port ratio is actually the opposite of what it should be. A better design would allocate more ports for iSCSI attachment and fewer to Fibre Channel, or at least an even ratio of iSCSI to Fibre Channel ports. As it is, if each Fibre Channel port is attached to a storage target, each would be grossly underutilized by the relatively few iSCSI hosts that could be installed on the 2Gbps Ethernet ports. In the example given in Figure 7-5, the 2 gigabits of iSCSI host access would be undersubscribed to the 12 gigabits (6 ports x 2Gbps Fibre Channel) available for storage access.

Figure 7-5. A fixed-port iSCSI-to-Fibre Channel storage switch

Other products in this class provide multiservice ports that can be configured for either Gigabit Ethernet or Fibre Channel on any port. This design presents a more difficult engineering challenge, but it overcomes the limitations of fixed-port solutions. As shown in Figure 7-6, a multiservice port design does not split out Fibre Channel and iSCSI connectivity but allows a more flexible assignment of devices. As the ratio of Fibre Channel and iSCSI end devices changes over time, individual ports can assume the appropriate personality to accommodate the change. This design, introduced by Nishan Systems in its IP storage switch, has enabled concurrent connectivity between any mix of Fibre Channel and IP storage end devices. As shown in Figure 7-6, iSCSI hosts have access to Fibre Channel targets, and Fibre Channel hosts can access iSCSI targets.

Figure 7-6. The multiservice port IP storage switch architecture pioneered by Nishan

In both of these examples, E_Port connectivity to existing Fibre Channel fabrics is provided. If the E_Port is properly implemented, this arrangement facilitates integration of new iSCSI products with Fibre Channel SAN assets.

For Fibre Channel-to-Fibre Channel extension, either iSCSI or iFCP can be used. The caveat is that not all storage applications will work if iSCSI is used as the intervening IP transport. EMC's Symmetrix Remote Data Facility (SRDF), for example, was originally engineered for Fibre Channel connectivity and expects FCP layer mapping between two storage arrays. The protocol conversion from Fibre Channel to iSCSI and back to Fibre Channel loses this necessary FCP layer association. Because iFCP preserves FCP commands, it is better suited to SRDF and other FCP-sensitive applications.

For Fibre Channel SAN extension over distance, iFCP gateways let you terminate E_Port connections at each site and thus avoid the problems associated with stretched fabrics, as discussed in Chapter 6. IP storage gateways that support both iFCP and iSCSI protocol conversion offer a single box solution for stable SAN extension and migration from Fibre Channel to iSCSI storage networking.