Have you deployed VAAI in your vSphere 5.x environment using newly created VMFS-5 datastores?
An issue has been discovered with the Atomic Test & Set (ATS) VAAI primitive (which is enabled by default) on ESXi 5.0 and ESXi 5.1 hosts using datastores which were created with VMFS-5
VMware has released the following KB article which clarifies the issue and recommends disabling ATS with VAAI enabled storage arrays:
ESXi 5.0 hosts fail to mount VMFS5 volumes that are formatted with ATS-only capabilities
Please note that according to the KB article:
“This issue is seen to occur in, but is not restricted to, EMC arrays Clariion CX4, NS, and VNX arrays”.