Admission control is part of vSphere High Availability (HA). It enforces and ensures availability in case of host failures. It guarantees that there is enough cluster capacity (memory or CPU) left for a HA failover by preventing VM power on actions that would violate that guarantee.
Since vSphere 6.5 there’s a dynamic calculation of minimum required resources, depending on your host number and host failures you want to tolerate.
Let’s start with an example: A cluster got two equal hosts and should tolerate one host failure. Admission control will make sure that neither CPU, nor memory load will exceed 50% of your total resources. If you lose one host there will be enough resources to restart VMs on the remaining host.
Let’s imagine you’re adding another two hosts to the cluster. The number of host failures to tolerate is still 1, but now dynamic resource calculation kicks in. With now four hosts, admission control will allow you to fill up the cluster to 75% before it will prevent VM power on.
That’s great. Because you just have to define your desired number of host failures to tolerate and HA admission control will dynamically calculate the allowed percentage of cluster resources to use. It works for adding and removing hosts likewise.
There’s an upgrade track from VCP6-NV to VCP-NV 2019 without taking an exam. It’s not easy to find this possibility. I will show all necessary steps to take and would like to thank Tim Burkhard (VMware Education Team), who pointed me into that direction.
First you go to VMware-Education and choose the VCP-NV 2019 track. Depending on your current certification status track requirements will be different. Only if you’re holding a current VCP6-NV certification, you’re able to upgrade without an exam. All others will have to pass the exam.
Choose VCP6-NV in the dropdown menu and read the requirements.
I usually get a lot of questions during trainings or in the process of vSAN designs. People ask me why there is a requirement for 30% of slack space in a vSAN cluster. If you look at it without going deeper, it looks like a waste of (expensive) resources. Especially with all-flash clusters it’s a strong cost factor. Often this slack space is mistaken as growth reserve. But that’s wrong. By no means it’s a reserve for future growth. On the contrary – it is a short term allocation space, needed by the vSAN cluster for rearrangements during storage policy changes.
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