VMware Path Selection Plug-ins (PSPs) are responsible for selecting a physical path for I/O requests.

The plug-ins are submodules of the VMware NMP. The NMP assigns a default PSP for each logical device based on the device type. You can override the default PSP.

Each PSP enables and enforces a corresponding path selection policy.

VMW_PSP_MRU — Most Recently Used (VMware)

The Most Recently Used (VMware) policy is enforced by VMW_PSP_MRU. It selects the first working path discovered at system boot time. When the path becomes unavailable, the host selects an alternative path. The host does not revert to the original path when that path becomes available. The Most Recently Used policy does not use the preferred path setting. This policy is default for most active-passive storage devices.

The VMW_PSP_MRU supports path ranking. To set ranks to individual paths, use the esxcli storage nmp psp generic pathconfig set command. For details, see the VMware knowledge base article at http://kb.vmware.com/kb/2003468 and the vSphere Command-Line Interface Reference documentation.

VMW_PSP_FIXED — Fixed (VMware)

This Fixed (VMware) policy is implemented by VMW_PSP_FIXED. The policy uses the designated preferred path. If the preferred path is not assigned, the policy selects the first working path discovered at system boot time. If the preferred path becomes unavailable, the host selects an alternative available path. The host returns to the previously defined preferred path when it becomes available again.

Fixed is the default policy for most active-active storage devices.

VMW_PSP_RR — Round Robin (VMware)

VMW_PSP_RR enables the Round Robin (VMware) policy. The policy uses an automatic path selection algorithm rotating through the configured paths. Round Robin is the default policy for many arrays. The policy can be used with both active-active and active-passive arrays to implement load balancing across paths for different LUNs. With active-passive arrays, the policy uses all active paths. With active-active arrays, the policy uses all available paths.

VMW_PSP_RR has configurable options that you can modify on the command line. To set these parameters, use the esxcli storage nmp psp roundrobin command. For details, see the vSphere Command-Line Interface Reference documentation.