When you unmount a file system, any pending writes are flushed to disk to ensure data integrity, the file system releases any resources in use, such as memory buffers and cache entries, and file system metadata is updated to reflect that the file system is no longer in use. The mount point where the file system was mounted is removed from the system namespace and the device is freed to be removed from the system.

A file system can't be unmounted if it's busy with open files, or if some process has a directory on the file system open.

Some options are available for the umount command to override default behavior. For example, you can perform a lazy unmount, which doesn't wait until all pending writes are flushed to disk, by using the -l option. This option can be useful when working with network mounted file systems that might have hung because of a network outage, and which might cause a significant delay when rebooting a system.

For more information, see the mount(8) and umount(8) manual pages.