5.4. Fibre Channel JBODs

A JBOD, or just a bunch of disks, is an enclosure with multiple Fibre Channel disk drives inserted into a common backplane. The backplane provides the transmit-to-receive connections that bring the assembled drives into a common Arbitrated Loop segment and bypass electronics that enable drives to be inserted or removed without disrupting the loop circuit. Since Fibre Channel drives provide primary and secondary NL_Ports for dual-loop attachment, the JBOD enclosure typically includes external interfaces for connecting two loops, as shown in Figure 5-5.

A JBOD brings the receive lead going to the first drive of a set and the transmit lead coming from the last drive of a set to a Fibre Channel interface, typically DB-9 copper. When the JBOD's interface is connected to a fibre channel hub or switch, the connection is not to a single Fibre Channel device but to multiple independent loop devices within the enclosure. An eight-drive JBOD, for example, appears as eight AL_PAs to the interconnect. If the connection is made to a switch, the switch port must be an FL_Port, since the downstream enclosure is a loop segment. If the connection is to an Arbitrated Loop hub, the population of the entire loop is increased by the number of drives in the JBOD. JBODs, unlike Fibre Channel RAIDs, have a direct impact on the topology to which they're attached.