When a VMFS-5 volume formatted on a VAAI capable device, it uses ATS locking, exclusively termed as
ATS only. To check if a VMFS volume is configured with ATS only, use this command:
# vmkfstools -Ph -v1 /vmfs/volumes/VMFS-volume-name
You see output similar to:
VMFS-5.54 file system spanning 1 partitions.
File system label (if any): ats-test-1
Mode: public ATS-only
When an ESX host discovers and registers new and existing devices, the VAAI plug-in module is loaded if the device is deemed to be VAAI capable during PSA device registration. In this particular case, ESX may fail to register device correctly with VAAI plug-in module as you are not reading the capacity of the volume correctly to apply the equivalent VAAI Plugin module. As a result, the VMFS5 volume may fail to mount.
This issue is seen to occur in EMC arrays Clariion CX4, NS, and VNX arrays, but is not restricted to these.