What Is Memory Ballooning?
It's like this you've got a bunch of stuff you need to remember, but your brain isn't big enough to hold it all. So, what do you do? You start taking out loans on your brain, but the bank charges interest! They keep coming back for more money. Okay, that's not how it works. Something like that is how memory ballooning works in virtualization platforms. When the host system reaches a threshold of utilized or active physical memory, it reclaims unused memory (e.g., RAM reserved by inactive VMs) and stores it in a fake balloon. As long as the host system maintains an active physical memory threshold, the balloon can artificially increase the amount of physical memory available to the host system. Once the physical memory threshold is exceeded and the host system begins reclaiming RAM from active VMs, the reclaimed RAM is stored in a second artificial balloon. VMs can draw from the second fake balloon until the physical memory threshold is reached. The first phony balloon remains reserved for the host system until its physical memory threshold is exceeded, at which time it can draw from the first balloon. The second artificial balloon remains reserved for the host system until its physical memory threshold is exceeded, when it can remove from the first balloon. When you need to reclaim memory, Hyper-V uses ballooning. That's when it sends a message to the guest operating system (installed on the virtual machine) saying, "Hey! You've got too much memory right now. Can you give some of it back?" The guest operating system then responds by returning some of its memory to the hypervisor. It is achieved through a balloon driver installed on the guest operating system, which the hypervisor communicates with when it needs to reclaim memory through ballooning.
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