In my Previous post, we have seen HA and its components. In this chapter, we are going to see Admission control and policies.
What is Admission Control?
vCenter Server uses admission control to
- ensure that sufficient resources are available in a cluster to provide failover protection
- ensure that virtual machine resource reservations are respected.
Admission Control Policy:
HA has three mechanisms as below to guarantee enough capacity is available to respect virtual machine resource reservations.
- Host Failures Cluster Tolerates
- Percentage of Cluster Resources Reserved
- Failover Hosts
Host Failures Cluster Tolerates
This option is historically speaking the most used for Admission Control. Most environments are designed with an N+1 redundancy and N+2 is also not uncommon. This Admission Control Policy uses “slots” to ensure enough capacity is reserved for failover, which is a fairly complex mechanism. Slots are based on VM level reservations and if reservations are not used a default slot size for CPU of 32 MHz is defined and for memory the largest memory overhead of any given virtual machine is used.
Pros:
- Fully automated (When a host is added to a cluster, HA re-calculates how many slots are available.)
- Guarantees failover by calculating slot sizes.
Cons:
- Can be very conservative and inflexible when reservations are used as the largest reservation dictates slot sizes.
- Unbalanced clusters lead to wastage of resources.
- Complexity for administrator from calculation perspective.
Percentage as Cluster Resources Reserved
The percentage based Admission Control is based on per-reservation calculation instead of the slots mechanism. The percentage based Admission Control Policy is less conservative than “Host Failures” and more flexible than “Failover Hosts”.
Pros:
- Accurate as it considers actual reservation per virtual machine to calculate available failover resources.
- Cluster dynamically adjusts when resources are added.
Cons:
- Manual calculations needed when adding additional hosts in a cluster and number of host failures needs to remain unchanged.
- Unbalanced clusters can be a problem when chosen percentage is too low and vSphere 6.x HA Deepdive Admission Control 102 resources are fragmented, which means failover of a virtual machine can’t be guaranteed as the reservation of this virtual machine might not be available as a block of resources on a single host.
Please note that, although a failover cannot be guaranteed, there are few scenarios where a virtual machine will not be able to restart due to the integration HA offers with DRS and the fact that most clusters have spare capacity available to account for virtual machine demand variance. Although this is a corner-case scenario, it needs to be considered in environments where absolute guarantees must be provided.
Specify Failover Hosts
With the “Specify Failover Hosts” Admission Control Policy, when one or multiple hosts fail, HA will attempt to restart all virtual machines on the designated failover hosts. The designated failover hosts are essentially “hot standby” hosts. In other words, DRS will not migrate virtual machines to these hosts when resources are scarce or the cluster is imbalanced.
Pros:
- What you see is what you get.
- No fragmented resources.
Cons:
- What you see is what you get.
- Dedicated failover hosts not utilized during normal operations.
That’s all for now folks. In my next chapter, will discuss about how slot and percentage mechanism are being calculated.
Source : Duncan Epping