I receive a lot of questions on why HA doesn’t work when virtual machines are not configured with VM-level reservations. If no VM-level reservations are used, the cluster will indicate a fail over capacity of 99%, ignoring the CPU and memory configuration of the virtual machines. Usually my reply is that HA admission control is not a capacity management tool and I noticed I have been using this statement more and more lately. As it doesn’t scale well explaining it on a per customer basis, it might be a good idea to write a blog article about it.
The basics
Sometimes it’s better to review the basics again and understand where the perception of HA and the actual intended purpose of the product part ways.
Let’s start of what HA admission control is designed for. In the availability guide the two following statement can be found: Quote 1:

“vCenter Server uses admission control to ensure that sufficient resources are available in a cluster to provide failover protection and to ensure that virtual machine resource reservations are respected.”

Let’s dive in the first quote and especially this statement: “To ensure that sufficient resources are available in a cluster” is the key element, and in particular the word sufficient (resources). What sufficient means for customer A, does not mean sufficient for customer B. As HA does not have an algorithm decoding the meaning of the word sufficient for each customer, HA relies on the customer to set vSphere resource management allocation settings to indicate the importance of resource availability for the virtual machine during resource contention scenarios.
As we are going back to the basics, lets have a quick look at the resource allocation settings that are used in this case, reservations and shares. A reservation indicates the minimum level of resources available to the virtual machine at all times. This reservation guarantees – or protect might be a better word –the availability of physical resources to the virtual machine regardless of the level of contention. No matter how high the contention in the system is, the reservation restricts the VMkernel from reclaiming that particular CPU cycle or memory page.
This means that when a VM is powered on with a reservation, admission control needs to verify if the host can provide these resources at all times. As the VMkernel cannot reclaim those resources, admission control makes sure that when it lets the virtual machine in, it can hold its promise of providing these resources all the time, but also checks if it won’t introduce problems for the VMkernel itself and other virtual machines with a reservation. This is the reason why I like to call admission control the virtual bouncer.
Besides reservation we have shares and shares indicates the relative priority of resource access during contention. A better word to describe this behavior is “opportunistic access”. As the virtual machine is not configured with a reservation, it provides the VMkernel with a more relaxed approach of resource distribution. When resource contention occurs, VMkernel does not need to provide the configured resources all the time, but can distribute the resources based on the activity and the relative priority based on the shares of the virtual machines requesting the resources. Virtual machines configured only with shares will just receive what they can get; there is no restrictive setting for the VMkernel to worry about when running out of resources. Basically the virtual machines will just get what’s left.
In the case of shares, it’s the VMkernel that decides which VM gets how many resources in a relaxed and very social way, where virtual machines configured with a reservation DEMAND to have the reservations available at all times and do not care about the needs of others.
In other words, the VMkernel MUST provide the resources to the virtual machine with reservation first and then divvy up the rest amongst the virtual machines who opted for a opportunistic distribution (shares).
How does this tie in with HA admission control?
The second quote gives us this insight:

“vSphere HA: Ensures that sufficient resources in the cluster are reserved for virtual machine recovery in the event of host failure.”

We know that admission control checks if there is enough resources are available to satisfy the VM-level reservation without interfering with VMkernel operations or VM-level reservations of other virtual machines running on that host. As HA is designed to provide an automated method of host failure recovery, we need to make sure that once a virtual machine is up and running it can continue to run on another host in the cluster if the current hosts fails. Therefor the purpose of HA admission control is to regulate and check if there are enough resources available in the cluster that can satisfy the virtual machine level reservations after a host failure occurs.
Depending on the admission control policy it calculates the capacity required for a failover based on available resources and still comply with the VMkernel resource management rules. Therefor it only needs to look at VM-level reservations, as shares will follow the opportunistic access method.
Semantics of sufficient resources while using shares-only design
In essence, HA will rely on you to determine if the virtual machine will receive the resources you think are sufficient if you use shares. The VMkernel is designed to allow for memory overcommitment while providing performance. HA is just the virtual bouncer that counts the number of heads before it lets the virtual machine in “the club”. If you are on the list for a table, it will get you that table, if you don’t have a reservation HA does not care if you decide to need to sit at a 4-person table with 10 other people fighting for your drinks and food. HA relies on the waiters (resource management) to get you (enough) food as quickly as possible. If you wanted to have a good service and some room at your table, it’s up to you to reserve.
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