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By Hugo Strydom, Managing Consultant, VMware Professional Services

In the first part we looked at how we collect network stats. In this part we will be looking at when and how LBT checks for load and the process of moving the VM traffic to another pNIC.

Let's first look at the detection and frequency. The vmkernel checks every 30sec the stats from the relevant pNIC's. The calculation will use the stats for the 30sec interval and get an average over the 30sec (to normalize the average and to eliminate spikes). If the bandwidth is above 75%, it will mark the pNIC as saturated.

It will look at RX and TX individually. Thus if RX is at 99% and TX is at 1% the avg is 50%, the calculation will take into account that the RX is above the 75% and thus also mark the pNIC as saturated.

If a pNIC have been marked as saturated, the vmkernel will not move any more traffic onto the saturated pNIC (except when a VM is powered on at initial placement, see part 1). VM traffic will only be moved off a saturated pNIC unless all pNIC have been marked as saturated (More on this in Part 3).

The process of moving a VM traffic from one pNIC to another is as follows :

  1. vmkernel detect pNIC is saturated based on 30sec calculation
  2. Calculation takes place to determine which VM's to move to which unsaturated pNIC
  3. VM traffic is moved to new pNIC

Note that when the VM traffic is moved there is no halt in traffic or VM world.

In part 3 we look at some scenarios and rules on how LBT will move VM traffic around.