In my last blog about vSphere HA basic concept, I explained the conceptual part of vSphere HA with some design tips.
Now, in the continuation of the same topic, I am going to explain Admission control Policy that we use to manage vSphere HA cluster for better resource utilization and management.
There are two types of Admission control policy that runs on top of vSphere HA
- Percentage based Admission control Policy
- Slot based Admission control Policy
In-short, slot based Admission control policy is more rigid and best suited for common / identical hardware based cluster whereas percentage based Admission control policy is more lenient and flexible policy that support all kind of clusters whether identical hardware based or of non-identical but with same processor family
What is Admission Control Policy?
It is the policy which would not let you start or power-on VM on top of ESXi host holding reserved capacity of resources for any disaster that may happen due to Hardware failure / Network disconnectivity. So, in a nutshell, Admission Control Policy (ACP) is used to keep a portion of hardware resources reserved (from pool of resources) for rainy days (Disastrous situation).
Below picture explains ACP at a glance
Formula to calculate and manage ACP
you can use formula for "Percentage" based ACP by looking into resources like
- Reserved CPU
- Reserved Memory
for Reserved CPU based resource reservation for ACP, you need to use Mhz / Ghz for a VM
Available Capacity - (Reserved CPU x number of VMs) / Total Capacity of CPU = %Percentage based ACP
For Example you got 2 VMs with 500 Mhz reserved for each VM out of 3.x Ghz CPU capacity per Host (holding Single Processor Single Socket) then formula above will be looking like as of below
3000Mhz - (500(Mhz) x 2 (VMs)) /3000 Mhz (Just to convert the remainder into percentage = 66% is the total Failover capacity now you can reserve how much in percentage for Admission control lets say 30% then remaining would be 36% left behind for your day 2 administration and consumption.
Similarly, We will be calculating Reserved Memory for VMs as of below formula
Total Memory of ESXi host - (Reserved Memory x Number of VMs) / Total Memory of ESXi host
For Example, there are 2 VMs with 1 GB Reserved memory Each and total amount of memory installed in ESXi host is 64 GB then below formula looks like
64 GB (Total Memory) - (1 GB (Reservered Memroy) x 2 (Number of VMs)) / 64 GB Install (Total Memory) = 96%
So, 96% is the failover capacity left behind that you can further calculate to reserve as Admission Control value like 30% reserved for ACP in this case the remaining capacity for Memory will be 66%.
Most of the times and most manageable calculation for Clusters for vsphere HA is "Percentage Based ACP"
Next Episode will be for Slot based ..!
Stay tuned!
