There can be multiple paths between a SCSI initiator and a SCSI target. Multiple paths between a host and a storage device are useful to provide more fault-tolerance as well as to improve system throughput. Multi-pathing software ensures that the same target volume is not seen as two separate LUNs by the host.
The multiple paths can be configured in active-active or active-standby modes. In the active-active mode, both paths are actively transferring data. In the active-standby mode, the standby path does not actively transfer data. Some multi-pathing software allows for dynamic load balancing of traffic between the multiple paths. Typically, the storage controller vendor also provides the multi-pathing driver that runs on the host, and this software is usually limited to only operating with the vendor’s storage devices. Software vendors are beginning to provide multi-pathing software that can interoperate with storage controllers from multiple storage vendors.