In vSphere, what does the Balloon driver rely on from the guest OS?

The Balloon driver operates by utilizing a mechanism wherein it communicates with the guest operating system to reclaim memory. In a situation where the host is experiencing memory pressure, the balloon driver instructs the guest OS to allocate memory to the balloon driver itself. This process effectively reclaims memory without needing to resort to swapping or requiring the guest OS to be aware of the hypervisor directly.

When the balloon driver is active, it "inflates" by allocating memory pages from the guest OS, which makes those pages unavailable to the guest for its own applications. This memory can then be used by other VMs running on the same host that may need additional resources. Because the balloon driver relies on the guest OS having available memory to reclaim, the memory availability is crucial for the function of the balloon driver.

Thus, the balloon driver fundamentally depends on the memory availability from the guest OS to perform its task of dynamically managing memory resources in response to the host's needs.