Vmware Slot Size Calculation

VMware slot sizes are an important topic if you're concerned with how many ESXi hosts are required to run your environment.

To begin this post, we need to understand what a slot is. A slot is the minimum amount of CPU and memory resources required for a single VM in an ESXi cluster. Slot size is an important concept because it affects admission control.

A VMware ESXi cluster needs a way to determine how many resources need to be available in the event of a host failure. This slot calculation gives the cluster a way to reserve the right amount of resources.

The slot has two parts, the CPU component and the memory component. Each of them has its own calculation. If there are no virtual machine resource reservations in the cluster, then the slot size (for ESXi 5 at least) is 32 Mhz for CPU and 0 MBs + overhead for memory. (I've used 80 MBs as my memory overhead in the examples)

In the example below we have 2 ESXi hosts that have the same amount of resources available for virtual machines. There are different sized VMs, but none of them have a reservation. Doing a quick calculation we can determine that 384 slots are available on each host.

We take the lower value between the CPU slot size and the memory slot size to determine the number of virtual machines that can be started up under admission control. So therefore we could safely start 384 machines on these ESXi hosts, have one fail, and have the other host start all of them.

(I should mention that it's unlikely that you could get 384 vms on one of these hosts. That would be a great consolidation ratio.)

What if you have a single large VM with a reservation, but the rest of the virtual machines are relatively small.

Let's look at the same environment, but this time let's make the larger VM have a reservation on it.

Admission control is going to tell us that only 6 slots are available on host B, so it will only allow 6 VMs on host A to be powered on. Since I'm using a simplistic diagram with only two hosts, we know that these VMs will still fit on the host but since we use the largest slot size to determine how much we can fail over admission control will stop us from powering on VMs.

Option 1 – Don't use reservations unless their is a good reason to do so.

Option 2 – We can manually configure the slot size on the cluster.

If you're curious about what the slot size is on your system, look at your cluster summary. There will be an item listed for slot size.

VMware slot sizes are an important topic if you're concerned with how many ESXi hosts are required to run your environment.

To begin this post, we need to understand what a slot is. A slot is the minimum amount of CPU and memory resources required for a single VM in an ESXi cluster. Slot size is an important concept because it affects admission control.

A VMware ESXi cluster needs a way to determine how many resources need to be available in the event of a host failure. This slot calculation gives the cluster a way to reserve the right amount of resources.

The slot has two parts, the CPU component and the memory component. Each of them has its own calculation. If there are no virtual machine resource reservations in the cluster, then the slot size (for ESXi 5 at least) is 32 Mhz for CPU and 0 MBs + overhead for memory. (I've used 80 MBs as my memory overhead in the examples)

In the example below we have 2 ESXi hosts that have the same amount of resources available for virtual machines. There are different sized VMs, but none of them have a reservation. Doing a quick calculation we can determine that 384 slots are available on each host.

We take the lower value between the CPU slot size and the memory slot size to determine the number of virtual machines that can be started up under admission control. So therefore we could safely start 384 machines on these ESXi hosts, have one fail, and have the other host start all of them.

(I should mention that it's unlikely that you could get 384 vms on one of these hosts. That would be a great consolidation ratio.)

What if you have a single large VM with a reservation, but the rest of the virtual machines are relatively small.

Let's look at the same environment, but this time let's make the larger VM have a reservation on it.

Admission control is going to tell us that only 6 slots are available on host B, so it will only allow 6 VMs on host A to be powered on. Since I'm using a simplistic diagram with only two hosts, we know that these VMs will still fit on the host but since we use the largest slot size to determine how much we can fail over admission control will stop us from powering on VMs.

Option 1 – Don't use reservations unless their is a good reason to do so.

Option 2 – We can manually configure the slot size on the cluster.

If you're curious about what the slot size is on your system, look at your cluster summary. There will be an item listed for slot size.

If you're in a situation where you think you need to add extra ESXi hosts to your cluster because you can't power on virtual machines without exceeding your admission control rules, take a look at your slot sizes first. It may save you some money on a host you don't really need.

What is SLOT?
As per VMWare's Definition,
'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.'
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 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 you understand the post.Thanks For Reading !!!!!