July 05, 2015

Most Recently Used (MRU) PSP

Most Recently Used (MRU) – VMW_PSP_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 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.”

"If the active path fails, then an alternative path will take over, becoming active. When the original path comes back online, it will now be the alternative path."

MRU
  • The ESXi host selects the path that it most recently used
  • This is the default used with a SAN that is set to Active/Passive
  • With this policy, a path is chosen and continues to be used so long as it does not fail
  • If it fails, another path is used, and it continues to be used so long as it does not fail, even if the previous path becomes available again.
  • 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
  • The policy is displayed in the client as the Most Recently Used (VMware) path selection policy
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.

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 generic deviceconfig set.


Policy
Active/Active Array Active/Passive Array
Round Robin No fail back Next path in round robin scheduling is selected
Most Recently Used Administrator action is required to failback 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

For optimal performance with the arrays for which ESXi defaults to MRU you might also consider the Round Robin path policy.

Reference:

No comments:

Post a Comment