VMware Multipathing policies in ESXi/ESX

Products

• VMware vSphere ESXi

Issue/Introduction

This article describes the various pathing policies that can be used with VMware ESXi/ESX. These are referred to as Path Selection Plug-ins (PSP) and are also called VMware Path Selection Policy(Policies).

Environment

VMware vSphere ESXi 5
VMware vSphere ESXi 6

VMware vSphere ESXi 7

VMware vSphere ESXi 8

Resolution

These pathing policies can be used with VMware ESXi/ESX:

Most Recently Used (MRU)	VMW_PSP_MRU	The 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 Logical Unit Numbers (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. Note: The preferred flag, while sometimes visible, is not applicable to the MRU pathing policy and can be disregarded.
Fixed (Fixed)	VMW_PSP_FIXED	The policy uses the designated preferred path flag if it is configured. Otherwise, it uses the first working path discovered at system boot time. If the ESXi/ESX host cannot use the preferred path or it becomes unavailable, the ESXi/ESX host 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) VMware	VMW_PSP_RR	The policy uses an automatic path selection, rotating through all available paths, enabling the distribution of load across the configured paths. Note: For logical units associated with Microsoft Cluster Service (MSCS) and Microsoft Failover Clustering virtual machines, the Round Robin pathing policy is supported only on ESXi 5.5 and later. For Active/Passive storage arrays, only the paths to the active controller will be used in the Round Robin policy. For Active/Active storage arrays, all paths will be used in the Round Robin policy. Best Practice for RR, see this Adjusting Round Robin IOPS limit from default 1000 to 1

​​​Notes:

• These pathing policies apply to VMware's Native Multipathing (NMP) Path Selection Plug-ins (PSP). Third-party PSPs have their own restrictions.
• Round Robin is not supported on all storage arrays. Please check with your array documentation or storage vendor to verify that Round Robin is supported and/or recommended for your array and configuration. Switching to an unsupported or undesirable pathing policy can result in connectivity issues to the LUNs (in a worst-case scenario, this can cause an outage).

Here are the steps to Change the VMware Path Selection Policy.

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 is detected by the NMP PSP.

Additional Information

• The Round Robin (RR) multipathing policies have configurable options that can be modified on the command line. Some of these options include:
  
  
  • As example for device using Round Robin (RR):
    • esxcli storage nmp device list

naa.<ID>
   Device Display Name: FreeNAS iSCSI Disk (naa.<ID>)
   Storage Array Type: VMW_SATP_ALUA
   Storage Array Type Device Config: {implicit_support=on; explicit_support=off; explicit_allow=on; alua_followover=on; action_OnRetryErrors=off; {TPG_id=1,TPG_state=AO}}
   Path Selection Policy: VMW_PSP_RR
   Path Selection Policy Device Config: {policy=rr,iops=1000,bytes=10485760,useANO=0; lastPathIndex=0: NumIOsPending=0,numBytesPending=0}
   Path Selection Policy Device Custom Config:
   Working Paths: vmhba65:C0:T2:L0
   Is USB: false

• A number of bytes to send along one path for this device before the PSP switches to the next path.
• A number of I/O operations to send along one path for this device before the PSP switches to the next path.
• A Number of useANO to enable the usage of Active-Non-Optimized paths.

• If the storage device is configured for Asymmetric Logical Unit Access (ALUA), Round Robin by default will use only the active optimized paths. Note: This behavior can be changed, but not recommended as it may result in performance degradation. 
   
• To change a LUN to use the Round Robin path policy, see Changing a LUN to use a different Path Selection Policy (PSP).
   
• To change newly provisioned LUNs to use Round Robin by default, see Changing the default pathing policy for new/existing LUNs.
   
• For more information, search for Multipathing Considerations and PSP in the vSphere 7 Documentation Center