HA admission control (Admission Control)
Use vSphere HA admission control to ensure that sufficient resources set aside in the host fails for the virtual machine recovery.
Admission Control impose some restrictions on the use of resources. Anything that may violate these limits will not be allowed. You may not allow operation example as follows:
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Power on virtual machines
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Migrating Virtual Machines
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Increase the virtual machine's CPU or memory reservation
VSphere HA based admission control is permitted and the cluster number of host failures can still ensure failover. It may be set in three main failure mode switching capacity:
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Percentage cluster resources
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Slot Strategy
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Dedicated failover host
https://docs.vmware.com/cn/VMware-vSphere/6.7/com.vmware.vsphere.avail.doc/GUID-53F6938C-96E5-4F67-9A6E-479F5A894571.html
DRS admission control (Admission Control)
When you try to open one or a set of virtual machines in a DRS enabled cluster power, vCenter Server performs admission control. It will check whether there are enough resources in the cluster to support the virtual machines.
If the cluster does not have enough resources to turn the power on a single virtual machine, or can not be opened in any attempt to start the power group of virtual machines, a message will be displayed. Otherwise, for each virtual machine, DRS generates a recommendation on which to run the virtual machine host and one of the following
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Automatically performs placement recommendation.
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Display user may then select to accept or override placement recommendations.
https://docs.vmware.com/cn/VMware-vSphere/6.7/com.vmware.vsphere.resmgmt.doc/GUID-AE4E1173-E3BF-40A6-8540-BD7289EA9F38.html
Resource allocation is access control (also called Admission Control Admission Control)
When the virtual machine is powered, the system checks not yet been reserved amount of CPU and memory resources. The system determines whether the available unreserved resources can guarantee the reservation of the virtual machine configuration (if any). This process is referred to as access control.
If there enough unreserved CPU and memory available, or there is no reservation, the virtual machine is powered on. Otherwise it will display aInsufficient Resourcescaveat.
https://docs.vmware.com/cn/VMware-vSphere/6.7/com.vmware.vsphere.resmgmt.doc/GUID-25AAD7FD-5579-4F7E-AB3A-60CF8D982A06.html
Resource Allocation Shares ----- personally think it is to take effect (if the share of resources a virtual machine is twice another virtual machine, the virtual machine when the two compete for resources, the first virtual machine in case of overload He is entitled to consume twice as much in the second virtual machine resources.)
Resource Allocation Reservation ----- reservation specifies the guaranteed minimum allocation of virtual machine resources.
Resource Allocation Limit ----- personally think it is only valid in the resource pool (the feeling of a virtual machine written official document, describe strange, so think for a resource pool)