Host machine memory is the hardware backing for guest virtual memory and guest physical memory. For best performance, the host machine memory should be at least slightly larger than the combined active memory of the virtual machines on the host. A virtual machine's memory size should be slightly larger than the average guest memory usage. Increasing the virtual machine memory size results in more overhead memory usage.


  • Memory usage is constantly high (94% or greater) or constantly low (24% or less).
  • Free memory consistently is 6% or less and swapping frequently occurs.


  • The host probably is lacking the memory resources required to meet the combined active memory size of all the running VMs.
  • Host machine memory resources are not enough to meet the demand, which leads to memory reclamation (like swapping) and degraded performance.


  • Verify that VMware Tools is installed on each virtual machine. The balloon driver is installed with VMware Tools and is critical to performance.
  • Verify that the balloon driver is enabled. The VMkernel regularly attempts to reclaim the unused virtual machine memory by ballooning and, if necessary, swapping. Generally, this does not impact virtual machine performance.
  • Reduce the memory space on the virtual machine, and correct the cache size if it is too large. This frees up memory for other virtual machines.
  • If the memory reservation of the virtual machine is set to a value much higher than its active memory, decrease the reservation setting so that the VMkernel can reclaim the idle memory for other virtual machines on the host.
  • Migrate one or more virtual machines to a host in a DRS cluster.
  • Add physical memory to the host.