An automounter is any program or software facility which automatically mounts filesystems in response to access operations by user programs. An automounter system utility (daemon under Unix), when notified of file and directory access attempts under selectively monitored subdirectory trees, dynamically and transparently makes local or remote devices accessible.
The automounter has the purpose of conserving local system resources and of reducing the coupling between systems which share filesystems with a number of servers. For example, a large to mid-sized organization might have hundreds of file servers and thousands of workstations or other nodes accessing files from any number of those servers at any time. Usually, only a relatively small number of remote filesystems (exports) will be active on any given node at any given time. Deferring the mounting of such a filesystem until a process actually needs to access it reduces the need to track such mounts, increasing reliability, flexibility and performance.
Frequently, one or more fileservers will become inaccessible (down for maintenance, on a remote and temporarily disconnected network, or accessed via a congested link). Administrators also often find it necessary to relocate data from one file server to another - to resolve capacity issues and balance the load. Having data mount-points automated makes it easier to reconfigure client systems in such cases.
These factors combine to pose challenges to older "static" management methods of filesystem mount tables (the fstab files on Unix systems). Automounter utilities address these challenges and allow sysadmins to consolidate and centralize the associations of mountpoints (directory names) to the exports. When done properly, users can transparently access files and directories as if all of their workstations and other nodes attach to a single enterprise-wide filesystem.
One can also use automounters to define multiple repositories for read-only data; client systems can automatically choose which repository to mount based on availability, file-server load, or proximity on the network.