6.7

For example, consider a virtual machine with a configured size of 1GB. When the guest operating system
boots, it detects that it is running on a dedicated machine with 1GB of physical memory. In some cases,
the virtual machine might be allocated the full 1GB. In other cases, it might receive a smaller allocation.
Regardless of the actual allocation, the guest operating system continues to behave as though it is
running on a dedicated machine with 1GB of physical memory.
Shares Specify the relative priority for a virtual machine if more than the reservation
is available.
Reservation Is a guaranteed lower bound on the amount of physical RAM that the host
reserves for the virtual machine, even when memory is overcommitted. Set
the reservation to a level that ensures the virtual machine has sufficient
memory to run efficiently, without excessive paging.
After a virtual machine consumes all of the memory within its reservation, it
is allowed to retain that amount of memory and this memory is not
reclaimed, even if the virtual machine becomes idle. Some guest operating
systems (for example, Linux) might not access all of the configured memory
immediately after booting. Until the virtual machines consumes all of the
memory within its reservation, VMkernel can allocate any unused portion of
its reservation to other virtual machines. However, after the guest’s
workload increases and the virtual machine consumes its full reservation, it
is allowed to keep this memory.
Limit Is an upper bound on the amount of physical RAM that the host can
allocate to the virtual machine. The virtual machine’s memory allocation is
also implicitly limited by its configured size.
Memory Overcommitment
For each running virtual machine, the system reserves physical RAM for the virtual machine’s reservation
(if any) and for its virtualization overhead.
The total configured memory sizes of all virtual machines may exceed the amount of available physical
memory on the host. However, it doesn't necessarily mean memory is overcommitted. Memory is
overcommitted when the combined working memory footprint of all virtual machines exceed that of the
host memory sizes.
Because of the memory management techniques the ESXi host uses, your virtual machines can use
more virtual RAM than there is physical RAM available on the host. For example, you can have a host
with 2GB memory and run four virtual machines with 1GB memory each. In that case, the memory is
overcommitted. For instance, if all four virtual machines are idle, the combined consumed memory may
be well below 2GB. However, if all 4GB virtual machines are actively consuming memory, then their
memory footprint may exceed 2GB and the ESXi host will become overcommitted.
Overcommitment makes sense because, typically, some virtual machines are lightly loaded while others
are more heavily loaded, and relative activity levels vary over time.
vSphere Resource Management
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