Suggestion: vc-hooks.el: Make vc-handled-backends more conservative?

["T. V. Raman" <raman@users.sourceforge.net>]

In vc-hooks.el, vc-handled-backends
is presently declared to handle a large number of backends:

(defcustom vc-handled-backends '(RCS CVS SVN SCCS Arch MCVS)

The last two --arch and MCVS have a particularly bad performance
penalty on systems that have NFS mounts in places other than /net
or /afs --- se variable vc-ignore-dir-regexp

Unless one discovers the above variable (vc-ignore-dir-regexp is
declared as a defvar, and not as defcustom)
nfs based systems take a heavy performance hit --- since function
vc-find-root recursively ascends the directory tree in attempting
to check if files are under version control.

Suggestions:

A) Possibly make vc-handled-backends more conservative by pruning
        it to have RCS CVS and SVN on it by default.

B)      Make vc-ignore-dir-regexp a customizable option via
defcustom,
        and add a pointer to it in the docstring for
        vc-handled-backends so that users discover its purpose

        In my case I chased it down by examining the source code
        after observing that emacs was making excessive nfs
        accesses during file load and save.

-- 
Best Regards,
--raman

      
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