Combining SCSI and IP for server clustering
By Tom Clark
Although many medium-and small-businesses could benefit from the high availability provided by server clustering, some lack the budget and expertise to implement server clusters based on Fibre Channel shared storage. A much-neglected area for server clustering is the accommodation of direct SCSI-attached storage and hosts over a storage network. If administrators could use their existing SCSI storage, hosts and LAN infrastructure to create a high availability SAN, they would provide their end-users with reliable data access, but without the cost and complexity of new storage and adapter acquisition.
Recently introduced IP storage bridges provide support for both SCSI initiators and SCSI targets. The SCSI devices are attached by traditional parallel cabling to the IP storage bridges, which are in turn connected to conventional Gigabit Ethernet LAN switches to create an IP SAN backbone. The Ethernet LAN provides the flexibility, distance and device population of networked storage and substitutes for the typically more expensive Fibre Channel fabric. SCSI hosts connected by IP storage bridges can access SCSI storage anywhere within the IP-routed network.
Since the SCSI hosts are connected to the IP SAN via parallel SCSI cabling, an external Ethernet or Fast Ethernet LAN connection is required for heartbeat messages that allow the servers to monitor the status of the cluster. This configuration puts advanced high availability solutions into the hands of customers who, due to budget or support constraints, have not yet implemented Fibre Channel SAN solutions. They also gain the benefit of non-disruptive addition of storage resources, a critical issue in high availability operation that cannot be resolved easily by conventional direct-attached SCSI.
About the author: Tom Clark is director of technical marketing at Nishan Systems. He is also a board member of the Storage Networking Industry Association (SNIA), co-chair of the SNIA Interoperability Committee, and author of Designing Storage Area Networks: A Practical Reference for Implementing Fibre Channel SANs . Another book, IP SANS: A Guide to iSCSI, iFCP, and FCIP Protocols for Storage Area Networks , is due out in November 2001.