“Failed to power on VM :
Could not power on VM : Admission check failed for memory resource See the VMware ESX Resource Management Guide for information on resource management settings. 
Group vm.80740: Cannot admit VM: Memory admission check failed. Requested reservation: 25046 pages”

It looks something that memory resource allocation problem. First I thought to check the resource pool settings But unfortunately, I am not using resource pools in my lab environment. After that I verified the memory information of the ESXi host in the configuration Tab -> Memory. Please refer the below snapshot. It tells me the memory available in the ESXi host for virtual  machines is 1641.0 MB. So nothing bad here.

I have only one virtual Machine named “vmwarearena” with 256 MB and also verified any virtual machine level reservation. I don’t have any.

After that, I have moved towards the Resource allocation tab of the ESXi host to check the available memory resource in the host. Found here is the culprit, It shows only 90 MB as available capacity of Memory.

Still you may think OK you have 90 MB available memory..Go ahead and power on but the problem is My VM “vmwarearena” has 256 MB of memory configured and with the memory overhead of 122.73 MB. SO to power on the virtual machine, i require 379 MB of memory( VM configured memory + memory overhead of the VM) i.e  (256 MB +123 MB = 379 MB).

The below steps are not supported procedure and under allocating the system resource  in the production environment will create a negative impact on the performance of the host and vm’s.  In a production environment, I would recommend to review the proper allocation of memory resources and  to increase the memory of host to accommodate and power on the failed virtual machines. 

Since this is Lab environment, I have performed the below steps to reduce the memory allocated to the ESXi system to power on my virtual machine.

Goto To ESXi host -> Configuration Tab-> Software -> System Resource Allocation

Click on Advanced, You will get the below screen.

Notice the above screenshot shows that ESXi host has 1641 MB memory reserved for the host use. I am going to reduce the memory reservation of ESXi host.

Select the node ” VIM” and under that click on “vmvisor“.  vmvisor has 974 MB of memory reservation. To edit this, Click on Edit Settings.

Reduce the Memory Reservation from 974 MB to 500 MB and click on OK. Now Memory reservation for “VMvisor” is only 500 MB. 

Now we have reduced the memory reservation of the host. Lets’ Try to Power on my VM “vmwarearena“.Wow!!!..It’s Powered on…..!!!!!!