Inside Linux Figure 12.1. A remote filesystem mounted
Inside Linux Figure 12.1. A remote filesystem mounted on a local directory. The Automounter and NFS Namespace The automounter must perform its magic somehow. The magic I am talking about is the capability to mount a filesystem and then later unmount the filesystem. The magic behind all this is called a map. The automounter uses maps to make the proper mounting associations. A map describes the association of the local pathname with the remote filesystem. The local pathname is actually called a mount point, or simply, mount. It is called a mount point because it is the “point” at which you “mount” a remote filesystem. The NFS namespace is the aggregation of mount points. A client maintains a mount point for each remote filesystem the client wishes to access. All accesses to the remote filesystem are made through the local mount point. The NFS system transparently shields the user from specifics of the remote filesystem. Referring to Figure 12.1, you can see that the mount point for the client is /mnt/data and the remote filesystem is the server:/usr/bill/data directory path. Both paths represent a root (start point), each from a different perspective. For the server side, the server:/usr/bill/data directory path is the starting point given to the NFS client. The client may only traverse down the hierarchy. For the client, the /mnt/data directory represents the logical start point for filesystem traversal. The word logical is used because you are not traversing a filesystem on your system. The key to NFS administration, if you consider automounter maps, is the maintenance of the automounter maps. The automounter maps should be stored in a central and well-known location. The Network Information Service (NIS), if available, can be used for this purpose. Otherwise, you can utilize the facilities of the Linux system files for map storage. Purpose of the Automounter The automounter has some amount of intelligence. As mentioned previously, the automounter automatically mounts remote filesystems as required. If a user is trying to access a filesystem that is not currently mounted, the automounter will attempt to mount the filesystem. The automounter uses information in the map to execute the mounting. The automounter will also automatically unmount a filesystem that has been idle for some time. I use idle here to mean a filesystem that has not been accessed over a period of time. You can set the minimum timeout for the automounter for automatic unmounting; the default is five minutes. If you set the timeout to zero, automatic unmounting is disabled. page 213
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