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GNU cfengine

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4.7 Using the automounter

The automounter is a daemon based service which replaces static mounting of NFS filesystems with a dynamical model. When the automounter is running, filesystems are mounted only when a user tries to access a file which resides on one of those filesystem. After a given period (usually five minutes) any filesystem which has not been accessed is unmounted. The advantage of this scenario is that hanging servers do not affect the behaviour of hosts which mount their filesystems, unless a specific file is being accessed. In both cases, filesystems must be exported in order to be mountable.

It is not the purpose of this section to explain the use of the automounter in detail, only to offer hints as to how cfengine can be used to simplify and rationalize automount configuration for the already initiated. Let us begin by comparing the behaviour of the automounter with the cfengine model for mounted filesystems.

The automounter is designed to be used together with a global configuration file, distributed by NIS (the network information service). As such, all hosts read the same configuration file. This makes it appear as though all hosts end up mounting every filesystem in the automount configuration database, but this is not so in practice because filesystems are only mounted if required. Thus a system which does not require a filesystem will not attempt to mount it. Moreover, the existence of a global configuration file does not affect which hosts have the right to mount certain filesystems (which is specified by exports or share on the relevant server), thus a request to mount a non-exported filesystem will result in an access denial. The automounter is configured locally on each host in files named `/etc/auto_master', `auto_direct' etc.

In the cfengine static mounting scheme, you define a list of binary and home servers. The filesystem table is modified on the basis of these decisions, and filesystems are only added if cfagent deems it appropriate to mount them on a given host. The idea here is to minimize the number of filesystems mounted to those which are known to be required. Again the issue of access permissions must be arranged separately. These filesystems are placed directly in `/etc/fstab', or the equivalent for your system.

From cfengine, you can use the automounter instead of the static mount model by

The automounter was created to solve certain problems which cfengine now solves (in the author's opinion) better. For example, the use of the `hosts' map in the automounter mounts filesystems like `/usr/local' on different (uniquely named) mountpoints for each host in order to avoid name space collisions. Using cfengine and a unique naming scheme, you can achieve the same thing more cleanly, without all of the gratuitous linking and unlinking which the automounter performs by itself. Moreover, the idea of a unique name-space is better practice and more in keeping with new global filesystem ideas such as AFS and DFS. The only advantage of the automounter is that one avoids the annoying error messages from hung servers about "NFS server not responding". In that respect, it seems sensible to use only direct mounts and a unique name space.

Some systems advocate grouping all users' login (home) directories under a common directory called `/home' or `users'. The automounter goes through all manner of contortions to achieve this task. If you use a unique naming scheme like the one advocated here, this is a trivial task. You simply arrange to mount or automount all user directories, such as

 
   /site/host/home1
   /site/host/home2
   ...

and then link them as follows:

 
   /home +> /site/host/home1
   /home +> /site/host/home2
   ...

Finally, you should be aware that the automounter does not like to be mixed with static mount and unmount operations. Automounted filesystems take priority over statically mounted filesystems, but the automounter can be confused by manually mounting or unmounting filesystems while it is running.


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