Mirror-mounted file systems are automatically unmounted if they are idle after a certain period of inactivity. The period is set by using the timeout parameter, which is used by the automounter for the same purpose.
If an NFS file system is manually unmounted, then any mirror-mounted file systems contained within it are also unmounted, if idle. If a mirror-mounted file system is active, the manual unmount fails as though that original file system were busy. However, a forced unmount is propagated through all the contained mirror-mounted file systems.
If a file system boundary is encountered within an automounted file system, a mirror mount occurs. When the automounter unmounts the parent file system, any mirror-mounted file systems within it are also automatically unmounted, if idle. If there is an active mirror-mounted file system, the automatic unmount does not occur, which preserves current automount behavior.