A VMFS datastore holds virtual machine files, directories, symbolic links, RDM descriptor files, and so on. The datastore also maintains a consistent view of all the mapping information for these objects. This mapping information is called metadata.
Metadata is updated each time you perform datastore or virtual machine management operations. Examples of operations requiring metadata updates include the following:
Creating, growing, or locking a virtual machine file
Changing a file's attributes
Powering a virtual machine on or off
Creating or deleting a VMFS datastore
Expanding a VMFS datastore
Creating a template
Deploying a virtual machine from a template
Migrating a virtual machine with vMotion
When metadata changes are made in a shared storage enviroment, VMFS uses special locking mechanisms to protect its data and prevent multiple hosts from concurrently writing to the metadata.