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Virtual memory ballooning is a computer memory reclamation technique used by a hypervisor to allow the physical host system to retrieve unused memory from certain guest virtual machines (VMs) and share it with others. Memory ballooning allows the total amount of RAM required by guest VMs to exceed the amount of physical RAM available on the host. When the host system runs low on physical RAM resources, memory ballooning allocates it selectively to VMs.
If a VM only uses a portion of the memory that it was allocated, the ballooning technique makes it available for the host to use. For example, if all the VMs on a host are allocated 8 GB of memory, some of the VMs will only use half the allotted share. Meanwhile, one VM might need 12 GB of memory for an intensive process. Memory ballooning allows the host to borrow that unused memory and allocate it to the VMs with higher memory demand.
The guest operating system runs inside the VM, which is allocated a portion of memory. Therefore, the guest OS is unaware of the total memory available. Memory ballooning makes the guest operating system aware of the host's memory shortage.
Virtualization providers such as VMware enable memory ballooning. VMware memory ballooning, Microsoft Hyper-V dynamic memory, and the open source KVM balloon process are similar in concept. The host uses balloon drivers running on the VMs to determine how much memory it can take back from an under-utilizing VM. Balloon drivers must be installed on any VM that participates in the memory ballooning technique.
Balloon drivers get the target balloon size from the hypervisor and then inflate by allocating the proper number of guest physical pages within the VM. This process is known as inflating the balloon; the process of releasing the available pages is known as deflating the balloon.
VM memory ballooning can create performance problems. When a balloon driver inflates to the point where the VM no longer has enough memory to run its processes within itself, it starts using another VM memory technique known as memory swapping. This will slow down the VM, depending upon the amount of memory to recoup and/or the quality of the storage IOPS delivered to it.