See man disparm for an explanation of the external specs of disparm.

I is not easy to arrange in a nfs file system that only one process
will change a file (the poolfile), and to ensure that this process
gets the most up-to-date version of the file.

In disparm this is accomplished by three steps:

 - create a lock directory. This is an atomic operation, if
   many processes are doing a 'mkdir' of the same directory,
   only one process can succeed.
 - create a hard link in that directory to the poolfile.
   If the link count of that link equals 2, it is considered
   successful, otherwize it is assumed that no exclusive
   access to the file is possible at this moment.
 - open de link with O_DIRECT, and read a block, to persuade the 
   system to get the latest version of the file.

If the lock cannot be obtained directly, it is tried some sconds later.