Vmware Slot Size
Vmware Slot Size Policy
Vsphere Slot Size Chart
Vsphere Ha Slot Size
Vsphere Ha Slot Size Calculation

I’ve been receiving a lot of questions around slot sizes lately. Although I point everyone to my HA Deepdive post not everyone seems to understand what I am trying to explain. The foremost reason is that most people need to be able to visualize it; which is tough with slot sizes. Just to freshen up an outtake from the article:

HA uses the highest CPU reservation of any given VM and the highest memory reservation of any given VM. If there is no reservation a default of 256Mhz will be used for the CPU slot and the memory overhead will be used for the memory slot!

If VM1 has 2GHZ and 1024GB reserved and VM2 has 1GHZ and 2048GB reserved the slot size for memory will be 2048MB+memory overhead and the slot size for CPU will be 2GHZ.

Now how does HA calculate how many slots are available per host?

It mentions that 'Notice that the memory slot size has been set to 1024MB. VM24 has a 4GB reservation set. Because of this VM24 spans 4 slots.' I would like to know if the VM did not have a reservation would it still use 4 slots because it had 4GB allocated to it.

Of course we need to know what the slot size for memory and CPU is first. Then we divide the total available CPU resources of a host by the CPU slot size and the total available Memory Resources of a host by the memory slot size. This leaves us with a slot size for both memory and CPU. The most restrictive number is the amount of slots for this host. If you have 25 CPU slots but only 5 memory slots the amount of available slots for this host will be 5.

The first question I got was around unbalanced clusters. Unbalanced would for instance be a cluster with 5 hosts of which one contains substantially more memory than the others. What would happen to the total amount of slots in a cluster of the following specs:

But we do not use vCPUs anymore to determine the slot size for CPU. The slotsize for CPU is determined by the highest reservation or 256MHz (vSphere 4.x and prior) / 32MHz (vSphere 5) if no reservation is set. However, vCPUs can have an impact on your slot it can have an impact on your memory slotsize.
Slot is a logical representation of CPU and memory. Depending on vSphere version, the default slot size is 0 MB of RAM and 256 MHz CPU (4.1 and earlier) or 0 MB of RAM and 32Mhz (5.0 and later). The admission control is performed by VMware HA with the following steps.
There are three AC policies in vSphere 6.5: Slot policy; As policy name states it uses slot size to calculate an amount of available resources for a failed host scenario. The slot size can be either defined automatically by using the largest reservations for RAM and vCPU. Alternatively, you can define a custom slot size.

Five hosts, each host has 16GB of memory except for one host(esx5) which has recently been added and has 32GB of memory. One of the VMs in this cluster has 4CPUs and 4GB of memory, because there are no reservations set the memory overhead of 325MB is being used to calculate the memory slot sizes. (It’s more restrictive than the CPU slot size.)

This results in 50 slots for esx01, esx02, esx03 and esx04. However, esx05 will have 100 slots available. Although this sounds great admission control rules the host out with the most slots as it takes the worst case scenario into account. In other words; end result: 200 slot cluster.

With 5 hosts of 16GB, (5 x 50) – (1 x 50), the result would have been exactly the same. To make a long story short: balance your clusters when using admission control!

The second question I received this week was around limiting the slotsizes with the advanced options das.slotCpuInMHz and/or das.slotMemInMB. If you need to use a high reservation for either CPU or Memory these options could definitely be useful, there is however something that you need to know. Check this diagram and see if you spot the problem, the das.slotMemInMB has been set to 1024MB.

Notice that the memory slotsize has been set to 1024MB. VM24 has a 4GB reservation set. Because of this VM24 spans 4 slots. As you might have noticed none of the hosts has 4 slots left. Although in total there are enough slots available; they are scattered and HA might not be able to actually boot VM24. Keep in mind that admission control does not take scattering of slots into account. It does count 4 slots for VM24, but it will not verify the amount of available slots per host.

To make sure you will always have enough slots and know what your current situation is Alan Renouf wrote an excellent script. This script reports the following:

Example Output:

Cluster : Production
TotalSlots : 32
UsedSlots : 10
AvailableSlots : 22
SlotNumvCPUs : 1
SlotCPUMHz : 256
SlotMemoryMB : 118

My article was a collaboration with Alan and I hope you find both article valuable. We’ve put a lot of time into making things as straight forward and simplistic as we possibly can.

VMware HA Slot is the default admission control option prior to vSphere 6.5. Slot Size is defined as the memory and CPU resources that satisfy the reservation requirements for any powered-on virtual machines in the HA cluster.This article is just to cover how the HA slots are calculated step by step not more than that. If you want to read the detailed information about HA, Please refer HA Deep Dive by Duncan Epping.

What is VMware HA Slots ?

As per VMWare’s Definition,

“VMware HA slots is a logical representation of the memory and CPU resources that satisfy the requirements for any powered-on virtual machine in the cluster.”

If you have configured reservations at VM level, It influence the HA slot calculation. Highest memory reservation and highest CPU reservation of the VM in your cluster determines the slot size for the cluster.

Here is the Example,

If you have the VM configured with the highest memory reservation of 8192 MB (8 GB) and highest CPU reservation of 4096 MHZ. among the other VM’s in the cluster, then the slot size for memory is 8192 MB and slot size for CPU is 4096 MHZ. in the cluster.

If no VM level reservation is configured , Minimum CPU size of 256 MHZ and memory size of 0 MB + VM memory overhead will be considered as CPU and Memory slot size.

Calculation for Number of HA Slots in cluster :-

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Once we got the Slot size for memory and CPU by the above method , Use the below calculation

Vmware Slot Size

Num of CPU Slots = Total available CPU resource of ESX or cluster / CPU Slot Size

Num of memory slots = Total available memory resource of ESX or cluster minus memory used for service console & ESX system / Memory Slot size

Let’s take a Example,

I have 3 host on the cluster and 6 Virtual machine is running on the cluster and Each host capacity as follows

RAM = 50 GB per Host Gambling age ontario canada.

CPU = 8 X 2.666 GHZ per host

Cluster RAM Resources = 50 X 3 = 150 GB – Memory for service console and system = 143 GB

Cluster CPU resources = 8 X 2.6 X 3 = 63 GHZ (63000 MHZ) of total CPU capacity in the cluster – CPU Capacity used by the ESX System = 60384 MHZ

Vsphere Slot Size