Vmware Slot Size Policy
As hopefully all of you know by now, the percentage based admission control policy does not work with fixed slot sizes. (More details to be found in the Availability Guide and the Best Practices white paper.)The percentage based admission control policy is a per virtual machine mechanism, hence the reason I always prefer this admission control.
Once your slot size has been determined, the number of slots on each host in the cluster is counted. The lower resource determines their limit - so if a host can fit 100x the CPU slot size in terms of CPU resource, but can only fit 30x the memory limit, the host has 30 slots. That number is added up for each host in the cluster. The guest memory size should not be set lower than the minimum recommendations of the operating system provider. The New Virtual Machine Wizard sets reasonable defaults for the memory size of a virtual machine, based on the type of the guest operating system and the amount of memory in the host computer. 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.
After you create a cluster, you can configure admission control to specify whether virtual machines can be started if they violate availability constraints. The cluster reserves resources so that failover can occur for all running virtual machines on the specified number of hosts.
The Admission Control page appears only if you enabled vSphere HA.
Procedure
- In the vSphere Client, browse to the vSphere HA cluster.
- Click the Configure tab.
- Select vSphere Availability and click Edit.
- Click Admission Control to display the configuration options.
- Select a number for the Host failures cluster tolerates. This is the maximum number of host failures that the cluster can recover from or guarantees failover for.
- Select an option for Define host failover capacity by.
Option
Description Cluster resource percentage Specify a percentage of the cluster’s CPU and memory resources to reserve as spare capacity to support failovers. Slot Policy (powered-on VMs) Select a slot size policy that covers all powered on VMs or is a fixed size. You can also calculate how many VMs require multiple slots. Dedicated failover hosts Select hosts to use for failover actions. Failovers can still occur on other hosts in the cluster if a default failover host does not have enough resources. Disabled Select this option to disable admission control and allow virtual machine power ons that violate availability constraints. - Set the percentage for the Performance degradation VMs tolerate. This setting determines what percentage of performance degradation the VMs in the cluster are allowed to tolerate during a failure.
- Click OK.
Your admission control settings take effect.
By Duncan Epping, Principal Architect, VMware
Yesterday I received a question on twitter:
Vmware Slot Size Policy Comparison
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.
Vmware Slot Size Policy Guide
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.
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 Policy Tool
I guess the answer to this question is: For CPU the number of vCPUs does not impact your slotsize, but for memory it may.