> Given that you need write access into /usr/src for the first run of
> the 'make obj' target, you can't really do a read only /usr/src tree,
> at least not all the time.  Exporting it read only after the symlinks
> have been made does help, as NFS then attempts to do better caching
> of contents and directory data.  (or it did on the SunOS 4.X machines
> where I first starting doing this)
> 
	Although people have warned me not to do it since union mounts first
	came out, what I do is have an NFS read-only source and union mount
	over a local filesystem. This has worked since the 90's for me...

	The source remains totally clean and the links, object files and
	executables stay on a local disk away from the RO source tree.

	The majority opinion is that this is a BAD(tm) way to do things, WHY?,
	and that I should just accept symlinks and stuff scribbled all over
	the master source directorys... B^(.

	Keep in mind that I have 4 arch's using the same NFS mount at the
	same time so I've encountered the symlink method not working quite
	right when I compile Amiga, Mac, x86 and SPARC all at the same time...

	Anyways, its POSSIBLE to use union mounts to do a read-only source
	but the core group has strongly advised me against doing so over
	the past 5+ years...

	-Rob