Ballooning – the lowdown

VMware Tools does many things for us as administrators, automating much of the resource management and monitoring we need so we can have confidence in managing the sprawling clusters of a private cloud.

Below are some of the benefits VMware Tools affords us:

  • Driver optimisation
  • Power settings management
  • Time synchronisation
  • Advanced memory management
  • VM Heartbeating

VMware Tools is available for every supported guest Operating System, and with the exception of certain appliances (which often come with 3rd Party tools installed anyway), installation of VMTools should be common practice.

In this article I am going to talk about one specific memory management technique: ballooning. Ballooning is a method for reclaiming host memory in times of contention, this allows for more workloads to run on a host without resorting to using swapping.

The VMTools service/daemon runs as any other process does in an Operating System, and can request resources the same as any other processes. When memory contention is high on a host, and VM allocated memory would likely need to be swapped to disk (the .vswp file, see here for more information on that), the hypervisor will send a request to the VMTools process running on its guest VMs to try and reclaim memory.

Often, software running in an OS will not release memory when it is done with it, this can lead to memory being tied up in the OS, and can therefore not be released to the hypervisor to return to the pool of available host memory.

When the VMTools process receives the signal, it will request up to 65% of the host memory be relinquished to it, at which time it will release the memory back to the hypervisor. Because of the way we allocate memory to VMs (vRAM), the VM often has more memory available to it than it requires and as such, most VMs will have memory which can be returned.

Ballooning is a normal part of the automated memory management processes which ESXi provides, high amounts of ballooning do not usually indicate a problem, although large amounts of ballooning activity could tell you that memory is overcommitted.

The only time ballooning can cause a problem is when the ballooning driver reclaims memory pages required by the guest OS, in this case it can lead to swapping which could lead to performance degradation.

tl;dr – ballooning is a normal part of vSphere’s memory management, assisting in pushing up consolidation ratios. Don’t worry about it, but it is good to be aware of what it does, and how it works.


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