ISCSI and SAN interconnections

This WAN overview focuses on how to connect SANs over a distance, as well as storage compatibility and data optimisation.

SAN data has traditionally existed as its own "island" of Fibre Channel technology -- accessible only to applications through the data centre. But the need to implement business continuity and disaster recovery plans has forced data centres to connect Fibre Channel SANs across the wide area network (WAN). The broad adoption of SoIP technologies, such as iSCSI, has further enhanced this connectivity. A storage manager creates an Ethernet...

storage fabric by using existing network hardware and tools to create an Ethernet storage fabric that can easily transfer iSCSI SAN data across TCP/IP-based networks, including the Internet. This overview examines the basic schemes used to connect SANs over a distance.

Local interconnections

Large corporations are finally joining small and medium-sized businesses (SMB) in deploying iSCSI SANs. Even for a corporation with a substantial investment in Fibre Channel infrastructure, iSCSI provides economical SAN connectivity for workgroups and remote offices. Many large companies are using iSCSI to connect a far larger number of servers to the SAN than would otherwise be affordable using Fibre Channel alone. Fibre Channel and iSCSI SANs are typically tied together so that each SAN can access the other, but it's necessary for a storage router to convert between the two protocols. One example is the SANbox 6140 Intelligent Storage Router from QLogic, which provides Fibre Channel and Ethernet ports so that both SAN fabrics can be connected together.

Such local interconnections do not involve a WAN, but the practice is so common that it's worth mentioning here to put other WAN-based connections into perspective.

FCIP

It is possible to connect two Fibre Channel SANs across an IP network using FCIP. This is a tunnelling protocol that takes the FCP packets containing SCSI (not iSCSI) commands and encapsulates them within TCP/IP packets that are then exchanged over an Ethernet network, such as the Internet.

FCIP requires the use of a dedicated storage router, such as the Model 735 FCIP Storage Router from Emulex. This type of device is located at each site, connecting the Fibre Channel fabric at each end to an Ethernet LAN and performing all the encapsulation functions in hardware at full T3 line speed. No special host bus adapters (HBA) or other equipment should be needed to accomplish FCIP connections between SANs. FCIP management is typically handled by the storage administrator.

IFCP

The iFCP is a variation on FCIP that ports FCP directly to IP -- there is no encapsulation because iFCP converts FCP packet addresses to corresponding IP addresses for transmission over the IP network or WAN. The iFCP packet is then reconverted back to FCP on the far end. However, iFCP is less popular today than FCIP.

As with local and FCIP connections, iFCP requires a dedicated storage router that supports the iFCP protocol, such as the M1620 from Brocade. One device is needed at each end of the connection to interface the Fibre Channel fabric to an IP network and perform the Fibre Channel-to-IP translation in real time. Another distinction: FCIP management is handled by the storage administrator and iFCP management handled by the network administrator.

Storage compatibility and data optimisation

Storage managers considering a storage router should evaluate its compatibility with existing storage platforms. For example, Brocade claims that its storage routers are qualified with all major storage platforms including EMC, Hitachi, HP, IBM and Sun-StorageTek. Lab testing will determine the level of compatibility.

Also consider the benefits of data optimisation functions available in the storage router. For example, the SANbox 6140 supports Ethernet jumbo frames that can increase the standard 1,500 byte packet to 16,000 bytes. By increasing the data payload in each packet, there are fewer handshakes to deal with, reducing latency and increasing the effective bandwidth. However, not all devices support jumbo frames properly. Testing will identify potential configuration problems early on.

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