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Under configuration check 'system resource allocation', someone could have got a bit carried away with how much memory ESXi reserves....
(For reference our 128GB hosts have got a reservation of 4206MB)
Alternatively, have you got any memory reservations set for VM's?
I guess ESXi might reserve the memory for a machine even if it's switched off - we don't have memory constraints so haven't got this set for me to check for you.
Might be a but in the metrics or something, give the host or vCenter a reboot and see if it clears up.
It's not stuck in a "pool" of resources is it?
As in you assigned it to a pool and it's reserved in there for when the VM's come online?
Not sure, but I think you need 1.5 times memory size space available on the ESX memory page disk. So if the Datastore configured as the default page memory location is full this may restrict the memory you can use?
Q: Why is total not the same as RAM size plugged in my memory slots?
A: This is because some memory range is not available for use. It is fine, if the difference is small. If the difference is big, there might be some hardware issue. Check your BIOS.