The SCSI passthrough over multipath is very commonly used in virtualization environments. The SCSI IO commands are forwarded from the virtual machine to the KVM hypervisor side storage via SG_IO. The KVM hypervisor side storage is always provied in a multipath configuration in order to facilitate high availability. Therefore, the developers may require a solution that allows them to quickly and efficiently setup an environment to diagnose and observe the behavior of virtio-scsi + QEMU + SG_IO + multipath + SCSI, e.g., how the path error is propagated to the virtual machine.

Source: LXer – Software-Only Setup to Diagnose QEMU SCSI Passthrough and Multipath Failover