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Availability is the most important requirement for any SAN, including iSCSI SANS, and it needs to be architected at the server, network and array level. At the network level, redundancy is achieved by deploying switches in pairs and leveraging Ethernet failover techniques like spanning tree and dynamic routing. At the server level, high availability is achieved by dual-connecting servers to Ethernet switches; with Microsoft Corp.'s 2.0 release of its iSCSI Initiator in 2005, multipath IO (MPIO) has enabled iSCSI hosts to redundantly connect to the iSCSI network.
"All our hosts run MPIO and are dual-connected into the iSCSI SAN," explains Kuljit Dharni, director of infrastructure at Babson College, Wellesley, MA. "While one of the onboard NICs is used for regular LAN access, the second onboard Ethernet port and an additional Intel PCI gigabit network adapter connect to the iSCSI SAN."
Redundancy options in iSCSI targets vary by vendor and product type. iSCSI gateway appliances, intelligent storage switches and server-based iSCSI targets are typically available in cluster configurations in which two devices run in active-active or active-passive mode. Some midrange storage arrays with iSCSI support, such as EMC Corp.'s Clariion CX3-20 and CX3-40, provide redundancy via dual-controller architectures. High-end arrays like the EMC Symmetrix DMX family are chassis based and redundancy is obtained by simply adding multiple iSCSI blades.
This was first published in February 2007