Locking in NFS version 4
With the NFS version 4 protocol, the support for file locking is part of the NFS protocol. The file locking support is structured so that an RPC callback mechanism is not required. This is a departure from the previous versions of the NFS file locking protocol, Network Lock Manager (NLM). The state associated with file locks is maintained at the server under a lease-based model. The server defines a single lease period for all states held by an NFS client. If the client does not renew its lease within the defined period, all state associated with the client's lease may be released by the server. The client may renew its lease with use of the RENEW operation or implicitly by use of other operations (primarily READ).
With the NFS version 4 protocol, a client user can choose to lock the entire file, or a byte range within a file. z/OS NFS client file locking requests can be managed with the llock(Y|N) parameter on the mount command or as an installation default.
z/OS NFS supports only advisory locking. Advisory locking is when the operating system keeps track of which files have been locked by which process, but does not prevent a process from writing to a file that is locked by another process. This means that a process can ignore an advisory lock if the process has adequate permission.