By Duncan Epping, Principal Architect, VMware

Yesterday I received a question on twitter:

Oct 06, 2009 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.
A previous post on my blog shows us how to get the slot size information for each cluster which has HA enabled, it also discusses what a slot size is and points to some further reading on Yellow-Bricks. Readers of the great HA and DRS book will know exactly what a slot size is, if you are unsure then I suggest you read my previous article or the HA and DRS book, it is important information to.

VMware High Availability (HA) is a utility that eliminates the need for dedicated standby hardware and software in a virtualized environment. VMware HA is often used to improve reliability, decrease downtime in virtual environments and improve disaster recovery/business continuity. Jun 10, 2014 Now I understand that Vmware takes the largest VM running, and uses this to calculate the slot size, that being said, is it assuming ALL VM's are basically using the same resources that the largest VM is taking up? Is it to my benefits to create a custom slot size.

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 ðŸ™‚

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

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 Ha Slot Size Explained 2

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