Vmware Slot Size Calculation

1. Vmware Slot Size Calculation Calculator
2. Vmware Slot Size Calculation Chart

What is VMware HA Slots ?

The Slot size is calculated by the CPU and Memory Reservations at the individual Virtual Machine level in the whole cluster, the largest VM's CPU and Memory reservation determines the slot size. For more information on the slot size calculations on the different versions. Use a calculator. They are all valid in some form or the other and have their specific purpose. I think it basically doesn’t matter if you’re sizing NSX, sizing vSphere or sizing another VMware product, if you use the methods above you get a infrastructure that is able to run your workload. Given this information, what is the correct slot size for the cluster? The CPU Reservation should be set to 32MHz and the memory reservation should be set to 32MB plus memory overhead. The CPU reservation should be set to 1 GHz and the memory reservation should be set to 0MB plus memory overhead. NiTRo June 28th, 2012. Hi Duncan, as this HA admition control calculation is based on vm reservation (rather than slot size for “host failure” policy), i guess this could lead to a serious overcommit if you don’t use vm reservation right? Slot size is calculated by comparing both the CPU and memory requirements of the virtual machines and selecting the largest. The largest CPU requirement (shared by VM1 and VM2) is 2GHz, while the largest memory requirement (for VM3) is 2GB. Based on this, the slot size is 2GHz CPU and 2GB memory.

“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 :-

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

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

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

I don’t have any memory or CPU reservation in my cluster, So, the default CPU slot size 256 MHZ and one of my Virtual machine is assigned with 8 vcpu and its memory overhead is 344.98 MB (which is the highest overhead among my 6 virtual machines in the cluster)

Let’s calculate the num of CPU & Memory slots

Num of CPU Slots = Total available CPU resource of cluster / CPUSlot size in MHZ

No of CPU Slots = 60384 MHZ / 256 MHZ = 235.875 Approx

Num of Memory Slots = Total available Memory resource of cluster / memory Slot Size in MB

Num of Memory Slots = 146432 / 345 = 424 Approx

The most restrictive number among CPU and Memory slots determines the amount of slots for this cluster. We have 235 slots available for CPU and 424 Slots available for Memory. So the most restrictive number is 235.

So, Total number of slots for my cluster is 235 Approx. Please find the below snapshot

Please refer my blog post on Understanding Total Slots, Used Slots & Available slots in VMware HA Slot to understand Total Slots, Used Slots & Available slots. I hope this article is helpful to you to understand HA slots calculation. Thanks for Reading!!. Be social and share it in social media, if you feel worth sharing it.

By Duncan Epping, Principal Architect, VMware

Yesterday I received a question on twitter:

Hi, to settle an argument in the office, if no reserves are in place, does number of vCPU’s affect slot size in vSphere 4? Thx 🙂

First of all, what is a slot? The availability guide explains it as follows

A slot is a logical representation of the memory and CPU resources that satisfy the requirements for any powered-on virtual machine in the cluster.

In other words a slot is the worst case CPU and Memory reservation scenario for any given virtual machine in a cluster. This slot is used when Admission Control is enabled and “Host Failures Tolerates” has been selected as the admission control policy. The total amount of available resources in the cluster will be divided by the slot size and that dictates how many VMs can be powered on without violating availability constraints. Meaning that it will guarantee that every powered on virtual machine can be failed over.

As said this slot is dictated by the worst case reservation for CPU and Memory. Prior to vSphere 4.0 we used the number of vCPUs to determine the slotsize for CPU as well. 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.

Vmware Slot Size Calculation Calculator

However, vCPUs can have an impact on your slot… it can have an impact on your memory slotsize. If no reservation is set anywhere HA will use the highest Memory Overhead in your cluster as the slot size for memory. This is where the amount of vCPUs come in to play, the more vCPUs you add to a virtual machine the higher will your memory overhead be.

Vmware Slot Size Calculation Chart

I guess the answer to this question is: For CPU the number of vCPUs does not impact your slotsize, but for memory it may.