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Unfortunately the ESX API does not expose local file system usage.
VMFS capacity can be monitored through the standard ESX agent monitoring and alerting on VMFS capacity thresholds can be accomodated accommodated via a plug-in available from uptime Support.
Alternatively, if your environment allows ssh access, a custom monitor that uses ssh to execute the vdf command on the ESX host could be used to monitor the root file system capacity.