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. For more information, see Change the Path Selection Policy.

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. Round Robin is the default policy for many arrays. It uses an automatic path selection algorithm rotating through the configured paths.
Both active-active and active-passive arrays use the policy to implement load balancing across paths for different LUNs. With active-passive arrays, the policy uses active paths. With active-active arrays, the policy uses available paths.
The latency mechanism that is activated for the policy by default makes it more adaptive. To achieve better load balancing results, the mechanism dynamically selects an optimal path by considering the following path characteristics:
  • I/O bandwidth
  • Path latency
To change the default parameters for the adaptive latency Round Robin policy or to disable the latency mechanism, see the Change Default Parameters for Latency Round Robin.
To set other configurable parameters for VMW_PSP_RR, use the esxcli storage nmp psp roundrobin command. For details, see the vSphere Command-Line Interface Reference documentation.