Automount Filesystem (`type:=auto')
===================================

   The "auto" filesystem type creates a new automount point below an
existing automount point.  Top-level automount points appear as system
mount points.  An automount mount point can also appear as a
sub-directory of an existing automount point.  This allows some
additional structure to be added, for example to mimic the mount tree of
another machine.

   The following options may be specified:

`cache'
     specifies whether the data in this mount-map should be cached.
     The default value is `none', in which case no caching is done in
     order to conserve memory.  However, better performance and
     reliability can be obtained by caching some or all of a mount-map.

     If the cache option specifies `all', the entire map is enumerated
     when the mount point is created.

     If the cache option specifies `inc', caching is done incrementally
     as and when data is required.  Some map types do not support cache
     mode `all', in which case `inc' is used whenever `all' is
     requested.

     Caching can be entirely disabled by using cache mode `none'.

     If the cache option specifies `regexp' then the entire map will be
     enumerated and each key will be treated as an egrep-style regular
     expression.  The order in which a cached map is searched does not
     correspond to the ordering in the source map so the regular
     expressions should be mutually exclusive to avoid confusion.

     Each mount map type has a default cache type, usually `inc', which
     can be selected by specifying `mapdefault'.

     The cache mode for a mount map can only be selected on the command
     line.  Starting Amd with the command:

          amd /homes hesiod.homes -cache:=inc

     will cause `/homes' to be automounted using the "Hesiod" name
     server with local incremental caching of all succesfully resolved
     names.

     All cached data is forgotten whenever Amd receives a `SIGHUP'
     signal and, if cache `all' mode was selected, the cache will be
     reloaded.  This can be used to inform Amd that a map has been
     updated.  In addition, whenever a cache lookup fails and Amd needs
     to examine a map, the map's modify time is examined.  If the cache
     is out of date with respect to the map then it is flushed as if a
     `SIGHUP' had been received.

     An additional option (`sync') may be specified to force Amd to
     check the map's modify time whenever a cached entry is being used.
     For example, an incremental, synchronised cache would be created
     by the following command:

          amd /homes hesiod.homes -cache:=inc,sync

`fs'
     specifies the name of the mount map to use for the new mount point.

     Arguably this should have been specified with the `${rfs}' option
     but we are now stuck with it due to historical accident.

`pref'
     alters the name that is looked up in the mount map.  If `${pref}',
     the "prefix", is non-null then it is prepended to the name
     requested by the kernel "before" the map is searched.

   The server `dylan.doc.ic.ac.uk' has two user disks: `/dev/dsk/2s0'
and `/dev/dsk/5s0'.  These are accessed as `/home/dylan/dk2' and
`/home/dylan/dk5' respectively.  Since `/home' is already an automount
point, this naming is achieved with the following map entries:

     dylan        type:=auto;fs:=${map};pref:=${key}/
     dylan/dk2    type:=ufs;dev:=/dev/dsk/2s0
     dylan/dk5    type:=ufs;dev:=/dev/dsk/5s0