Vsphere Slot Size
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. 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. In vCenter Server 4.0, the slot size is shown in the vSphere Client on the Summary tab of the cluster. For more information, see the VMware Availability Guide. In vCenter Server 4.1 and 5.x, the slot size information is shown in the Summary tab of the cluster by clicking Advanced Runtime Info. The vSphere Availability guide states “A slot is a logical representation of memory and CPU resources. By default, it is sized to satisfy the requirements for any powered-on virtual machine in the cluster.” - In simple terms a slot can be consumed by a single virtual machine, but a virtual machine may consume more than one slot. So in my case I have 32872 Mhz and 40963MB of total capacity. My slot size is 32Mhz/80MB. Maximum number of CPU slots I can have is 328.25. Maximum number of RAM slots 40963/80=512.0375 which defines the number of total slots as HA chooses the most restrictive number (a worst case scenario).
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Vsphere Ha Slot Size Calculation
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Vsphere Ha Slot Size
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vSphere HA slot sizes are used to calculate the number of VMs that can be powered on in an HA cluster with “Host failures cluster tolerates” selected. The slots size is calculated based on the size of reservations on the VMs in the cluster. HA Admission Control then prevents new VMs being powered on if it would not leave any slots available should a host fail.
The slot size for a cluster can be seen by going to the Summary Page for the cluster and clicking the “Advanced Runtime Info” link in the HA box.
If none of the VMs have CPU or RAM reservations, a default of 256MHz and 0GB is used.
The slots per host is derived by taking the total available CPU/RAM for the host and dividing by the slot size. Some CPU is reserved for the system so it will usually be a little lower than the full amount. So a host with 2xquad-core 2.4GHz CPUs (total 19.2GHz) and no VM CPU or RAM reservations has 73 slots and will only allow 73 VMs to be powered on if the cluster has two hosts and is set to protect against a single host failure.
Obviously this allows a very minimal amount of resource for each VM, so either reservations should be set for each VM, or slots size can be manually adjusted (see the VMware vSphere Availability Guide (pdf) for full details).
Note that the slot size is used for admission control calculations only. It has no direct effect on the resources available to VMs should an HA event occur.
Vsphere 6.5 Slot Size
There is a VMware Knowledgebase article (1010594) which has some details of the difference in VI3 and vSphere 4.x.