5.8 Fibre Channel-to-SCSI Bridges

When a new technology encroaches on an older one, there is an opportunity for transitional products to bridge the gap. Fibre Channel-to-SCSI bridges perform this function for Fibre Channel SANs by allowing legacy SCSI devices to participate in storage networking. Fibre Channel-to-SCSI bridges normally provide one or two Fibre Channel interfaces for SAN attachment, and two to four SCSI ports for SCSI disk arrays or tape backup subsystems. In addition to this physical and transport conversion, the Fibre Channel-to-SCSI bridge translates serial SCSI-3 protocol to the appropriate SCSI protocol required by the legacy devices.

Vendors of these products like to call them "routers" instead of bridges, largely because of the poor reputation LAN bridge products acquired in the 1980s and their association with disruptive network broadcast storms. The Fibre Channel-to-SCSI products, however, do not actually route data at layer 3; instead, they simply convert one form of SCSI to another. In other words, they provide a bridging function.

The most common application for Fibre Channel-to-SCSI bridges is to support legacy tape backup subsystems, as illustrated in Figure 5-15. Existing SCSI tape backup and library systems represent a substantial investment whose replacement with Fibre Channel devices may be prohibitively costly. Because tape backup is an integral requirement for any storage network, Fibre Channel-to-SCSI bridges may therefore both preserve your investment in tape subsystem hardware and satisfy the need to optimize the tape backup process itself.

Figure 5-15. A Fibre Channel-to-SCSI bridge supporting SCSI-attached tape

graphics/05fig15.gif

Placing a tape subsystem behind a Fibre Channel-to-SCSI bridge also removes ownership of the subsystem from an individual server. Because the tape subsystem is now addressable by any server on the storage network, all servers can share what was previously a dedicated resource. The separation of a tape subsystem from server ownership is taken a step further by other products discussed later in this chapter.

The role of Fibre Channel-to-SCSI bridges will decline as legacy devices are eventually replaced by SAN-enabled systems. Given the large population and investment in traditional SCSI tape and disk products in use, however, Fibre Channel-to-SCSI bridges will continue to be useful options for SAN design for some time to come.



Designing Storage Area Networks(c) A Practical Reference for Implementing Fibre Channel and IP SANs
Designing Storage Area Networks: A Practical Reference for Implementing Fibre Channel and IP SANs (2nd Edition)
ISBN: 0321136500
EAN: 2147483647
Year: 2003
Pages: 171
Authors: Tom Clark

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