Since the markup backend was made the default during 2.5 it has allowed
any GConf subtree to be stored in a single file rather than every dir
having its own %gconf.xml file. E.g. /apps/metacity looks like:

  /apps/metacity/%gconf.xml
  /apps/metacity/general/%gconf.xml
  /apps/metacity/global_keybindings/%gconf.xml
  /apps/metacity/keybinding_commands/%gconf.xml
  /apps/metacity/window_keybindings/%gconf.xml

With 2.5 or later, you can do:

  $> gconf-merge-tree ~/.gconf/apps/metacity

and it will create

  /apps/metacity/%gconf-tree.xml

which is basically all the %gconf.xml files merged into a single file. The
original %gconf.xml files are left in tact.

From that point on, all changes made with the markup backend are made to
%gconf-tree.xml only, but if you log into a machine using the old XML backend
it will use the original %gconf.xml files.

We haven't made the backend use this feature by default yet, since we don't
want to break compatibility with older version of GNOME. However, we should
figure out what the default behaviour *should* be if we switched it on and
allow sysadmins to switch this on themselves if the compatibility with older
versions is not an issue.

Some thoughts:

  + The way it would work would be the backend would have a list of directories
    which should be merged. When loading any of those dirs, if it notices that
    the dir doesn't have a %gconf-tree.xml it would flag the dir to be saved in
    a merged file. When creating a dir, it just uses %gconf-tree.xml.

  + We need to decide whether that list should be hardcoded or configurable
    somehow. If configurable, we need to decide whether it should be 
    configurable per-backend-source or just system-wide.

  + Thinking about it, I think we should make it configurable both system-wide
    and per-backend. This would allow us to use merged files in the defaults
    database, allow the sysadmin to decide whether users should have merged
    files and allow that to be overidden by the user.
  
  + Good defaults would be that each subdir under /apps, /desktop, 
    /schemas/apps and /schemas/desktop should be merged. The defaults and
    mandatory database would use these, but homedirs wouldn't.

  + Individual apps may want to configure how their prefs are merged. E.g.
    panel would want each profile in a single file rather than everything
    in a single file (doesn't matter much since we don't use profiles really)
    and evo might want addressbook, calendar and mail preferences in separate
    files.

The cheesy config file might look like this by default:

<xmlbackend_config>
  <merging>

    <!-- Whether all sources should merge by default -->
    <merge>false</merge>

    <!-- List of dirs which should be merged -->
    <mergedirs>
      <dir>/apps/*</dir>
      <dir>/apps/panel/profiles/*</dir>
      <dir>/desktop</dir>
      <dir>/schemas/apps/*</dir>
      <dir>/schemas/desktop</dir>
    </mergedirs>

    <!-- Individually control each source -->
    <sources>
      <source>
        <location>/etc/gconf/gconf.xml.defaults</location>
        <merge>true</merge>
      </source>

      <source>
         <location>/etc/gconf/gconf.xml.mandatory</location>
         <merge>true</merge>
      </source>

      <!-- Uncomment to enable merging in homedirs
      <source>
         <location>$(HOME)/.gconf</location>
         <merge>true</merge>
      </source>
        -->
    </sources>

  </merging>
</xmlbackend_config>

This is rapidly feeling like its way to over-engineered and difficult
for people to understand.