Memory ballooning is a memory management technique that allows a physical host to take advantage of unused memory on its guest virtual machines (VMs).
When a host OS is short on physical memory, the hypervisor polls balloon drivers installed on the guests to request available memory. The guests respond by using page-reclaiming algorithms to determine which pages are available and can be assigned to the balloon drivers. The process of assigning available pages to the driver is known as inflating the balloon. Releasing available pages is known as deflating the balloon.
In a positive sense, memory ballooning allows that the total amount of RAM required by guest virtual machines to exceed the amount of physically available RAM on the host. A problem can occur, however, when a balloon driver inflates to the point where the guest VM no longer has the memory it needs to run processes within the guest. In such a scenario, the guest operating system will start swapping things out to deal with its memory issues. This swap activity can negatively affect performance, depending upon the amount of memory to recoup and/or the quality of the storage IOPS delivered to the VM.
Other memory-reclaiming techniques include memory overcommit, memory paging, memory mirroring and transparent page sharing.
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