Multipathing policies in ESX/ESXi 4.x and ESXi 5.x

(KB Article from VMware http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1011340)

These pathing policies can be used with VMware ESX/ESXi 4.x and ESXi 5.x:

• Most Recently Used (MRU) — Selects the first working path, discovered at system boot time. If this path becomes unavailable, the ESX/ESXi host switches to an alternative path and continues to use the new path while it is available. This is the default policy for Logical Unit Numbers (LUNs) presented from an Active/Passive array. ESX/ESXi does not return to the previous path when if, or when, it returns; it remains on the working path until it, for any reason, fails.
  
  Note: The preferred flag, while sometimes visible, is not applicable to the MRU pathing policy and can be disregarded.
• Fixed (Fixed) — Uses the designated preferred path flag, if it has been configured. Otherwise, it uses the first working path discovered at system boot time. If the ESX/ESXi host cannot use the preferred path or it becomes unavailable, ESX/ESXi selects an alternative available path. The host automatically returns to the previously-defined preferred path as soon as it becomes available again. This is the default policy for LUNs presented from an Active/Active storage array.
• Round Robin (RR) — Uses an automatic path selection rotating through all available paths, enabling the distribution of load across the configured paths.
  

  • For Active/Passive storage arrays, only the paths to the active controller will used in the Round Robin policy.
  • For Active/Active storage arrays, all paths will used in the Round Robin policy.

  
  Note: This policy is not currently supported for Logical Units that are part of a Microsoft Cluster Service (MSCS) virtual machine.
• Fixed path with Array Preference — The VMW_PSP_FIXED_AP policy was introduced in ESX/ESXi 4.1. It works for both Active/Active and Active/Passive storage arrays that support ALUA. This policy queries the storage array for the preferred path based on the arrays preference. If no preferred path is specified by the user, the storage array selects the preferred path based on specific criteria.
  
  Note: The VMW_PSP_FIXED_AP policy has been removed from the ESXi 5.0 release and VMW_PSP_MRU became the default PSP for all ALUA devices

Notes:

• These pathing policies apply to VMware's Native Multipathing (NMP) Path Selection Plugins (PSP). Third party PSPs have their own restrictions.
• Switching to Round Robin from MRU or Fixed is safe and supported for all arrays, Please check with your vendor for supported Multipathing policies for your storage array. Switching to a unsupported pathing policy can cause an outage.

Warning: VMware does not recommend changing the LUN policy from Fixed to MRU, as the automatic selection of the pathing policy is based on the array that has been detected by the NMP PSP.

Additional Information

The Round Robin (RR) multipathing policies have configurable options that can be modified at the command-line interface. Some of these options include:

• Number of bytes to send along one path for this device before the PSP switches to the next path.
• Number of I/O operations to send along one path for this device before the PSP switches to the next path.