Multipathing is a technique that lets you efficiently and reliably use more than one physical path to transfer data between the host and an external storage array.

VMware supports different types of storage architectures as follows:

• Active-active storage system: All controllers (or all ports) are active; that means all the paths are active unless a path fails. In this storage, it is possible to access a LUN simultaneously through all the storage paths that are available without significant performance degradation.
• Active-passive storage system: Usually one controller is active (on a specific LUN) and the second is passive (but could be active on another LUN). If access through the active storage port fails, one of the passive storage processors can be activated by the servers accessing it.
• Asymmetrical storage system: Supports Asymmetric Logical Unit Access (ALUA), where all ports could be active, but with different levels of access per port. With ALUA, hosts can determine the states of target ports and prioritize paths; some of the active paths are primary and others are secondary.
• Virtual port storage system: Supports access to the storage services through a single virtual port. Virtual port storage systems are active-active storage devices, but hide their multiple connections through a single port; for example with iSCSI it is possible to work with a single virtual IP instead of each IP of each port, or for FC it is possible to build a virtual WWN (if switches support NPIV). These storage systems handle port failovers and connection balancing transparently (transparent failover).

Depending on the storage type, you need specific multipath criteria. VMware ESXi has three main types of path selection policies (provided by NMP) described as follows:

• Fixed: The host uses the designated preferred path if it has been configured. Otherwise, it selects the first working path discovered at system boot time. This is the default policy for most active-active storage and there is also Fixed_AP that extends fixed functionality to active/passive and ALUA storage.
• Most Recently Used (MRU): The host selects the path that it used most recently. When the path becomes unavailable, the host selects an alternative path and does not revert to the original path when that path becomes available again. This is the default policy for most active-passive storage.
• Round Robin (RR): The host uses an automatic path selection algorithm, rotating through all active paths when connecting to active-passive arrays, or through all available paths when connecting to active-active arrays. This is the default for a number of arrays and can be used with both active-active and active-passive arrays to implement load balancing across multiple paths.

If the path selection for your storage is not recognized correctly or you want to change the default, you must use PSA-related commands for NMP and SATP. Refer to your storage vendor documentation and best practices for VMware vSphere.