For each storage device, the ESX/ESXi host sets the path selection policy based on the claim rules defined in the /etc/vmware/esx.conf file.

By default, VMware supports the following path selection policies. If you have a third-party PSP installed on your host, its policy also appears on the list.

Fixed (VMW_PSP_FIXED)

The host always uses the preferred path to the disk when that path is available. If the host cannot access the disk through the preferred path, it tries the alternative paths. The default policy for active-active storage devices is Fixed.

Fixed AP (VMW_PSP_FIXED_AP)

Extends the Fixed functionality to active-passive and ALUA mode arrays.

Most Recently Used (VMW_PSP_MRU)

The host selects the path that it used 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.

Round Robin (VMW_PSP_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 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 performance in terms of throughput, such as I/O per second, megabytes per second, or response times.

Path Policy Effects summarizes how the behavior of host changes, depending on the type of array and the failover policy.

Path Policy Effects

Policy/Controller

Active/Active

Active/Passive

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 using the preferred path. This 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.

Fixed AP

For ALUA arrays, VMkernel picks the path set to be the preferred path.

For both A/A and A/P and ALUA arrays, VMkernel resumes using the preferred path, but only if the path-thrashing avoidance algorithm allows the fail-back.