Managing Path Policies

For each storage device managed by NMP, and not PowerPath, an ESXi host uses a path selection policy. If you have a third-party PSP installed on your host, its policy also appears on the list.

Supported Path Policies

The following path policies are supported by default.

Policy Description

Policy	Description
`VMW_PSP_FIXED`	The host uses the designated preferred path, if it has been configured. Otherwise, the host selects the first working path discovered at system boot time. If you want the host to use a particular preferred path, specify it through the vSphere Web Client, or by using esxcli storage nmp psp fixed deviceconfig set. See Changing Path Policies. The default policy for active-active storage devices is `VMW_PSP_FIXED`. Note: If the host uses a default preferred path and the path's status turns to Dead, a new path is selected as preferred. However, if you explicitly designate the preferred path, it will remain preferred even when it becomes inaccessible.
`VMW_PSP_MRU`	The host selects the path that it used most recently. When the path becomes unavailable, the host selects an alternative path. The host does not revert back to the original path when that path becomes available again. There is no preferred path setting with the MRU policy. MRU is the default policy for active-passive storage devices. The `VMW_PSP_MRU` ranking capability allows you to assign ranks to individual paths. To set ranks to individual paths, use the esxcli storage nmp psp generic pathconfig set command. For details, see the VMware knowledge base article 2003468.
`VMW_PSP_RR`	The host uses an automatic path selection algorithm that rotates through all active paths when connecting to active-passive arrays, or through all available paths when connecting to active-active arrays. Automatic path selection implements load balancing across the physical paths available to your host. Load balancing is the process of spreading I/O requests across the paths. The goal is to optimize throughput performance such as I/O per second, megabytes per second, or response times. `VMW_PSP_RR` 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 paths for different LUNs.

VMW_PSP_FIXED

The host uses the designated preferred path, if it has been configured. Otherwise, the host selects the first working path discovered at system boot time. If you want the host to use a particular preferred path, specify it through the vSphere Web Client, or by using esxcli storage nmp psp fixed deviceconfig set. See Changing Path Policies.

The default policy for active-active storage devices is VMW_PSP_FIXED.

Note: If the host uses a default preferred path and the path's status turns to Dead, a new path is selected as preferred. However, if you explicitly designate the preferred path, it will remain preferred even when it becomes inaccessible.

VMW_PSP_MRU

The host selects the path that it used most recently. When the path becomes unavailable, the host selects an alternative path. The host does not revert back to the original path when that path becomes available again. There is no preferred path setting with the MRU policy. MRU is the default policy for active-passive storage devices.

The VMW_PSP_MRU ranking capability allows you to assign ranks to individual paths. To set ranks to individual paths, use the esxcli storage nmp psp generic pathconfig set command. For details, see the VMware knowledge base article 2003468.

VMW_PSP_RR

The host uses an automatic path selection algorithm that rotates through all active paths when connecting to active-passive arrays, or through all available paths when connecting to active-active arrays. Automatic path selection implements load balancing across the physical paths available to your host. Load balancing is the process of spreading I/O requests across the paths. The goal is to optimize throughput performance such as I/O per second, megabytes per second, or response times.

VMW_PSP_RR 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 paths for different LUNs.

Path Policy Effects

The type of array and the path policy determine the behavior of the host.

Policy	Active/Active Array	Active/Passive Array
Most Recently Used	Administrator action is required to fail back after path failure.	Administrator action is required to fail back after path failure.
Fixed	VMkernel resumes using the preferred path when connectivity is restored.	VMkernel attempts to resume by using the preferred path. This action can cause path thrashing or failure when another SP now owns the LUN.
Round Robin	No fail back.	Next path in round robin scheduling is selected.