The latency mechanism is activated for the Round Robin path selection policy by default. The mechanism considers I/O bandwidth and path latency to select an optimal path for I/O. When using the latency mechanism, the Round Robin policy can dynamically select the optimal path and achieve better load balancing results.

You can change the default parameters of the latency mechanism, or disable the mechanism.

Prerequisites

Set the path selection policy to Round Robin. See Change the Path Selection Policy.

Procedure

  1. Configure the latency mechanism by using the following command. 
     esxcli storage nmp psp roundrobin deviceconfig set --type=latency --device=device ID
    The command takes the following parameters:
    Parameter Description
    -S|--num-sampling-cycles=sampling value When --type is set to latency, this parameter controls how many I/Os to use to calculate the average latency of each path. The default value of this parameter is 16.
    -T|--latency-eval-time=time in ms When --type is set to latency, this parameter controls the frequency at which the latency of paths is updated. Default is 3 minutes.
  2. Verify whether the latency Round Robin and its parameters are configured correctly. 
     esxcli storage nmp psp roundrobin deviceconfig get --device=device ID
    or 
     esxcli storage nmp device list --device=device ID

    The following sample output shows the path's configuration: 

     Path Selection Policy: VMW_PSP_RR
   Path Selection Policy Device Config: {policy=latency,latencyEvalTime=180000,samplingCycles=16,curSamplingCycle=16,useANO=0; CurrentPath=vmhba1:C0:T0:L0: NumIOsPending=0,latency=0}

What to do next

To disable the latency mechanism, in the Advanced System Settings for your host, change the Misc.EnablePSPLatencyPolicy parameter to 0.