Product Announcements

Admission control: "used slots" exceeds "total slots"

By Duncan Epping, Principal Architect.

On the VMTN forum today someone asked how it was possible that the “used slots” exceeded the “total slots”. This is what their environment showed in vCenter:

HA Advanced Runtime Info:
Slot size                          4000Mhz
                                   4 vCPUs,
Total Slots in Cluster             16
Used Slots                         66
Available Slots                    0
Total Powered on vms in Cluster    66
Total Hosts in cluster             2
Total good host                    2

You can imagine this person was very surprised to see this. How can you have 66 slots used and only 16 total slots available in your cluster? There are two possible explanations:

  1. Admission Control is disabled
  2. A reservation was set on a virtual machine after all virtual machines were powered on, skewing the numbers

Let’s tackle number 1 first. If you disable admission control the vSphere UI will still show the slot size and the number of slots etc, it just won’t do anything with it…

With regards to the second explanation it might be easier to give an example:

Just imagine you have 2 hosts and HA does its calculations and you have 100 slots available. You power-on 100 VMs. Now you set a reservation on a VM, this reservation will change the slotsize. HA does it calculations again based on this new slotsize. This will result in only 25 slots available based on this new slotsize. However you already used 100 slots. In other words, now you have 25 totals slots and your used is 100.