Most Recently Used (MRU): The VMW_PSP_MRU policy selects the first working path, discovered at system boot time. If this path becomes unavailable, the ESXi/ESX host switches to an alternative path and continues to use the new path while it is available. This is the default policy for Logical Unit Numbers (LUNs) presented from an Active/Passive array. ESXi/ESX does not return to the previous path if, or when, it returns; it remains on the working path until it, for any reason, fails.
Note: The preferred flag, while sometimes visible, is not applicable to the MRU pathing policy and can be disregarded.
Fixed (Fixed): The VMW_PSP_FIXED policy uses the designated preferred path flag, if it is configured. Otherwise, it uses the first working path discovered at system boot time. If the ESXi/ESX host cannot use the preferred path or it becomes unavailable, the ESXi/ESX host selects an alternative available path. The host automatically returns to the previously defined preferred path as soon as it becomes available again. This is the default policy for LUNs presented from an Active/Active storage array.
Round Robin (RR): The VMW_PSP_RR policy uses an automatic path selection, rotating through all available paths, enabling the distribution of load across the configured paths.
For Active/Passive storage arrays, only the paths to the active controller will be used in the Round Robin policy.
For Active/Active storage arrays, all paths will be used in the Round Robin policy.
Note: For logical units associated with Microsoft Cluster Service (MSCS) and Microsoft Failover Clustering virtual machines, the Round Robin pathing policy is supported only on ESXi 5.5 and later.
Fixed path with Array Preference: The VMW_PSP_FIXED_AP policy was introduced in ESXi/ESX 4.1. It works for both Active/Active and Active/Passive storage arrays that support Asymmetric Logical Unit Access (ALUA). This policy queries the storage array for the preferred path based on the array's preference. If no preferred path is specified by the user, the storage array selects the preferred path based on specific criteria.
Note: The VMW_PSP_FIXED_AP policy is removed from ESXi 5.x. For ALUA arrays in ESXi 5.0, the MRU Path Selection Policy (PSP) is normally selected but some storage arrays need to use Fixed. To check which PSP is recommended for your storage array, see the Storage/SAN section in the VMware Compatibility Guide or contact your storage vendor.
These pathing policies apply to VMware's Native Multipathing (NMP) Path Selection Plug-ins (PSP). Third-party PSPs have their own restrictions. Round Robin is not supported on all storage arrays. Please check with your array documentation or storage vendor to verify that Round Robin is supported and/or recommended for your array and configuration. Switching to an unsupported or undesirable pathing policy can result in connectivity issues to the LUNs (in a worst-case scenario, this can cause an outage).