The locking hierarchy allows an application to obtain a lock in preparation for modifying the file system and yet allows other processes access to the drive. In this way, other processes are only denied access to the volume when absolutely necessary.
An application should perform direct disk write operations only within a level 3 lock. To obtain a level 3 lock, an application must make three calls to the lock volume function — first to obtain a level 1 lock, then a level 2 lock, and finally a level 3 lock. After obtaining the level 3 lock, the application can safely access the volume directly.
To release the lock on a volume, an application must call the appropriate unlock volume function the same number of times that the corresponding lock volume function was called. Each call to the unlock volume function decrements the lock level. For example, a level 3 lock returns to a level 2 lock, and the system processes any blocked read operations. A level 2 lock returns to a level 1 lock, and the system processes any blocked write operations or new file mappings. A call to the unlock volume function on a level 1 lock, however, releases the lock on the volume and allows other processes to obtain the lock.
Exclusive volume locks are owned by a process, not a thread. If necessary, a multithreaded application can obtain a level 1 lock in one thread, a level 2 lock in another, and a level 3 lock in yet another.
Even though the system may block or fail new file mappings, other processes are allowed to write to files through existing file mappings, because file mappings cannot be resized. Writing to an existing file mapping only changes the contents of the memory-mapped file, it does not cause changes to the file system.