memory ballooning

Memory ballooning is a virtual memory management technique used to free unused memory. 

Having multiple virtual machines (VMs) on a single physical server requires virtual memory management techniques to control resource sharing and to prevent shortages. Some processor chipsets use hardware to offload a portion of the virtual memory management work by creating two layers of page tables, the data structure that provides the mapping between virtual addresses and physical addresses. The layers, however, make it difficult for the hypervisor to see a VM's memory contents, how much memory that VM requires or whether the VM is consuming too much memory.

Balloon drivers, which are installed in each VM, transfer the memory shortage from the host (where the shortage exists) to the VM. The hypervisor alerts the balloon driver of low memory instances and instructs it to inflate, which locks a set of unused memory in the VM. The hypervisor can then reassign the physical memory to another VM.  This swap activity can potentially impact performance depending upon the amount of memory to recoup and/or the quality of the storage IOPS delivered to the VM. In a VMware environment, the balloon driver only activates when memory becomes scarce, so it’s best to have no ballooning activity at all. In a Windows Server environment, the balloon driver allocates RAM to the VM on-demand.

 

Contributor(s): Julia Anderson
This was last updated in May 2013
Posted by: Margaret Rouse
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