Simply pulling two FC cables from a host and re-cabling incorrectly (think a switch of two HBA cables) or a surprise reconfiguration of a storage system target could lead to a situation where the potential for data corruption is ripe. While it may not be reasonably possible to prevent data corruption in this scenario (think I/O already queued to target devices underneath the multipath target driver), the prudent course of action may be to try to prevent data corruption whenever the potential is discovered using reasonable means. Besides such pilot error as described above, the pre-requisites for the cable switch scenario include at least (1) having the cable disconnect and subsequent re-connect events occur while no user or path checker initiated I/O occurs to paths using the switched cables and (2) the target-side connectivity for the two cables is asymmetrical. Possible solutions could involve (1) detecting fiber channel disconnect hotplug events and acting upon them or (2) modifying multipath checker functions to verify a path's UID remains consistent in addition to verifying path connectivity. EMC's multipathing product PowerPath uses the latter approach for testing paths to SCSI logical units on both Symmetrix and CLARiion storage systems. Although using a single I/O for path testing may not be possible for many storage systems, the PowerPath checker function for both Symmetrix and CLARiion storage determines the UID for the SCSI logical unit from reply (or sense) information returned by a single test I/O and fails a path if the UID is not consistent.
