[Gllug] A Couple of Conceptual Questions

[Dylan Brewis]

On Saturday 24 November 2001 1:45 am, you wrote:
> On Fri, 23 Nov 2001, Dylan Brewis spake:
> > This is what I want to set up:
> >
> > On a network of 3 machines, each machine has a local home directory, but
> > the users need to occasionally log on to another. Is it safe to mount the
> > local home directory (which resides on its own on hda2) onto /localhome
> > and then use autofs to mount /localhome/<user> into /home/<user> along
> > with the other two from NFS? Both /localhome and /home would be mounted
> > (or exist) in /.
> >
> > What would happen if such a scheme led to recursion?

Actually, I've decided to mount the local /home/* directories from a separate 
partition now, so I can automount a partition direct to /home/user (only one 
local user) and that removes the issue, I think.

So, what are the very bad things that can happen? I can see that recursive 
mounting would at least confuse the filesystem so that file operations went 
foobar, but what about circular or recursive links? The main reason I ask is 
so I can diagnose the problems if they occur!

>
> Very bad things. Don't do it that way.
>
> Luckily there is a way to do it that is simultaneously easier to
> implement, leads to identical images on all machines and avoids cycles.
>
> Stick the actual home directories in, say,
> /home/.{FQDN-of-machine-exporting-fs}; on each machine exporting some
> home directories, one of those is local; the rest are NFS mounts from
> the other machines.
>
> /home on all the machines contains nothing but a bunch of symlinks
> pointing into /home/.{FQDN}.
>
> Bingo.
>
> (Note that any such scheme means that all the machines are mounting FSen
> from all the others; I have learned to my cost that this makes the
> network very hard to bring up all at once. It's a good idea to use -o bg
> when mounting the NFS filesystems on such a network, so that even if one
> machine doesn't come up, it doesn't stall all the rest!
>
> --- thanks to Kieran for beating me into looking into robustness... ye
> gods networks can get brittle easily if you don't constantly look out
> for such things...)


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