Whether a VSA cluster supports memory overcommitment for virtual machines that run on VSA hosts, depends on the version of ESXi you use.

If all cluster members run ESXi version 5.1, the VSA cluster supports memory overcommitment.

However, the VSA cluster that includes ESXi version 5.0 hosts does not support memory overcommitment. In this type of cluster, if a virtual machine becomes overcommitted, it starts to swap to its per-virtual-machine swapfile. VMX swapping is enabled by default and swapping to VSA datastores can make the cluster unstable, and as a result, virtual machines might begin powering off.

To prevent virtual machine downtime, do not overcommit memory in the VSA cluster that uses ESXi 5.0 hosts. For each virtual machine in the VSA cluster, reserve the same amount of memory that is allocated to that virtual machine and disable the machine from swapping to the VSA datastores. Such a configuration ensures that all virtual machines running on an ESXi host in the VSA cluster do not use more memory than that available on the host, and do not attempt to perform VMX swapping.

Important

After you upgrade to vSphere 5.1 and VSA 5.1, optionally reverse these changes to enable memory overcommitment for virtual machines.

For a VSA cluster that has HA enabled, the cluster reserves additional host memory to support the restart of virtual machines from a failed peer host. In a 2-member VSA cluster, HA reserves 50 percent of the reserved memory of each host for the restart of a failed-over virtual machine. Similarly, in a 3-member VSA cluster, HA reserves 33 percent of the reserved memory of each host for the restart of a failed-over virtual machine.

If you attempt to power on a virtual machine that exceeds the reserved memory threshold, the operation fails. Due to the memory threshold of an ESXi host, it is possible that not all virtual machines can be restarted on a running host in the event of a host failure.