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As usual with the release of each new vSphere version, VMware continues to provide the best availability and resource management features for business critical application workloads. vSphere 6.5 also added new and improved features with vSphere HA and DRS. In this article , we discuss in detail about vSphere 6.5 HA admission Control policy. Below are the some of the new features available with vSphere 6.5 High Availability.

• Simplified vSphere HA Admission Control

In vSphere 6.5, along with vSphere HA, We now have vCenter 6.5 HA also. Lot of new improvements to vSphere 6.5 HA Admission control has been made for simplified administration. HA Admission control is used to set aside a calculated amount of resources that are used in the event of a host failure. We will discuss in detail about What’s new with vSphere 6.5 HA Admission control.

Dec 09, 2011  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. The Number of ESXi hosts 2. The total CPU sockets per host 3. Ghz per Core 4. Physical Cores per CPU Socket (Not Hyper-threads) 5. Total RAM per host 6. Total number of VMs 7. The Desired Availability Level (N+x) Next enter the total number of vCPUs and vRAM assigned (or expected to be) assigned to VMs 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.

vSphere 6.5 HA Admission Control

Host Failure Cluster Tolerates

This option allows you to simply define the number of ESXi hosts tolerate for failures. vSphere HA will automatically calculate a percentage of resources to set aside by applying the “Percentage of Cluster Resources” (Default option in vSphere 6.5) admission control policy. Resources required for failover capacity is now directly related to the Host failures cluster tolerates option. In this Example, I have 2 ESXi hosts in the cluster and I have Configured “Host failures cluster tolerates” value as “1”. HA automatically reserves 50% of Memory and CPU for the failover capacity. Similarly, 4-host cluster and FTT=1 will calculate a 25% reservation. Until vSphere 6.5, HA Slot policy is the default admission control policy. With vSphere 6.5, default admission control policy is changed to “Cluster resource Percentage”.

if you have added or removed ESXi hosts in the cluster, percentage of failover capacity will be automatically recalculated.

You have option to Override the failover capacity by the number of host failures cluster tolerates by selecting the Override option and specify % for CPU and Memory.

Performance Degradation Tolerance

The reserved capacity used by admission control ensures that all configured reservations will continue to be honored after a host failure. If the reservations are not used in some environments, the performance of the cluster could be impacted. This new setting called “VM resource reduction event threshold” defines how much of a performance impact is tolerated and will issue a warning if the consumed resources are higher than the reserved resources.

0% – Raises a warning if there is insufficient failover capacity to guarantee the same performance after VM’s restart.

100% -Warning is disabled.

HA Slot Policy

You can also have option to choose “Slot Policy” . This is the default option prior to vSphere 6.5. Slot Size is defined as the memory and CPU resources that satisfy the reservation requirements for any powered-on virtual machines in the HA cluster. Take a look how HA slot Size is calculated. You have 2 options under Slot Policy:

1. Cover All powered-on Virtual Machines: It calculates the slot size based on the maximum CPU/Memory reservation and overhead of all powered-on virtual machines in the cluster
2. Fixed Slot Size: You can explicitly specify the fixed slot size

Dedicated Failover Hosts

This option allows you to define an dedicated ESXi host in the cluster as failover hosts for the HA cluster. That dedicated failover host will not run virtual machines unless vSphere HA needs to recover from a failed host. Click on + symbol to add dedicated failover hosts for the vSphere HA cluster.

That’s it. I hope this article will help you to clear with new options available with vSphere 6.5 admission control policy. Thanks for reading!!. Be social and share it in social media, if you feel worth sharing it.

You can configure vSphere HA to tolerate a specified number of host failures. With the Host Failures Cluster Tolerates admission control policy, vSphere HA ensures that a specified number of hosts can fail and sufficient resources remain in the cluster to fail over all the virtual machines from those hosts.

Vmware Slot Size

With the Host Failures Cluster Tolerates policy, vSphere HA performs admission control in the following way:

Calculates the slot size.

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.

Slot size is comprised of two components, CPU and memory.

If your cluster contains any virtual machines that have much larger reservations than the others, they will distort slot size calculation. To avoid this, you can specify an upper bound for the CPU or memory component of the slot size by using the das.slotcpuinmhz or das.slotmeminmb advanced attributes, respectively. See vSphere HA Advanced Attributes.

After the slot size is calculated, vSphere HA determines each host's CPU and memory resources that are available for virtual machines. These amounts are those contained in the host's root resource pool, not the total physical resources of the host. The resource data for a host that is used by vSphere HA can be found by using the vSphere Client to connect to the host directly, and then navigating to the Resource tab for the host. If all hosts in your cluster are the same, this data can be obtained by dividing the cluster-level figures by the number of hosts. Resources being used for virtualization purposes are not included. Only hosts that are connected, not in maintenance mode, and that have no vSphere HA errors are considered.

Vmware Cluster Setup

The maximum number of slots that each host can support is then determined. To do this, the host’s CPU resource amount is divided by the CPU component of the slot size and the result is rounded down. The same calculation is made for the host's memory resource amount. These two numbers are compared and the smaller number is the number of slots that the host can support.

The Current Failover Capacity is computed by determining how many hosts (starting from the largest) can fail and still leave enough slots to satisfy the requirements of all powered-on virtual machines.

Vmware Cluster Server

When you select the Host Failures Cluster Tolerates admission control policy, the Advanced Runtime Info link appears in the vSphere HA section of the cluster's Summary tab in the vSphere Client. Click this link to display the following information about the cluster:

The way that slot size is calculated and used with this admission control policy is shown in an example. Make the following assumptions about a cluster:

The cluster is comprised of three hosts, each with a different amount of available CPU and memory resources. The first host (H1) has 9GHz of available CPU resources and 9GB of available memory, while Host 2 (H2) has 9GHz and 6GB and Host 3 (H3) has 6GHz and 6GB.

Vmware Esxi Cluster

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.

The cluster has one available slot (the six slots on H2 and H3 minus the five used slots).