I just wanted to point out a couple of changes for HA in vSphere 5.0 with regards to admission control. Although they might seem minor they are important to keep in mind when redesigning your environment. Lets just discuss each of the admission control policies and list the changes underneath.
- Host failures cluster tolerates
Still uses the slot algorithm. Major change here is that you can have a value larger than 4 hosts. The 4 host limit was imposed by the Primary/Secondary node concept. As this constraint has been lifted it is now possible to select a value up to 31. So in the case of a 16 host cluster you can set the value to 15. (Yes you could even set it to 31 as the UI doesn’t limit you but that wouldn’t make sense would it…) Another change is the default slotsize for CPU. The default slotsize used to be 256MHz. This has been decreased to 32MHz.
- Percentage as cluster resources reserved
This admission control policy has been overhauled and it is now possible to select a percentage for both CPU and Memory separately. In other words you can set CPU to 30% and Memory to 25%. The algorithm hasn’t changed and this is still my preferred admission control policy!
- Specify Failover host
Allows you to select multiple hosts instead of just 1. So for instance in an 8 host cluster you can specify two as designated failover hosts. These hosts will not be used during normal operations, keep this in mind!
For more details on admission control I would like to refer to the HA deepdive (not updated to 5.0 yet) or my book on vSphere 5.0 Clustering which contains many examples of how to correctly set the percentage for instance.