Preventing Shutdown on Active NFS Servers?

Ed Almos asks: "Like many Slashdot Readers, I run a small network at home with a server and a number of desktops. The server holds all our files as NFS shares and doubles as a desktop machine should the need arise. Problems however occur if the server is shut down whilst there are NFS shares in use, the minimum disruption is a crashed desktop and a couple of times I have had to deal with corrupted files. Does anyone know of a way to prevent shutdown of a machine if someone else has drives mounted to its NFS shares ? I have already explored use of the /etc/shutdown.allow file but all this does is determine who can kill the machine. The minimal solution would be something similar to a Microsoft Windows system, where a request to shutdown brings up a warning window that there are users connected to the system, but I am not sure how to achieve this on a Linux system. Ideally I would like to prevent shutdown of a system with active NFS shares altogether, or at least until the user has unmounted and logged off the network."

Comments

But...

[Trbmxfz]

Not quite an answer to the article's question, but...

Theoretically, once the NFS server has crashed, shouldn't all clients simply freeze until the server is back? On all systems I used, this was the observed behavior, and it is quite useful actually: it seems to avoid all data loss problems (under conditions). When the NFS gets reachable, all running program go on executing as if nothing had happened.

A solution to the original problem, though, would be: tell all user that the NFS machine is to be powered on constantly.

You've got something wrong...

[Whip]

If your NFS server rebooting, shutting down, or crashing causes any problem but temporarilly 'hung' clients, you have something wrong.

NFS is explicitly designed to be stateless, precisely to allow it to function across server reboots, crashes, and other fun. If your clients are crashing, or getting back corrupted data, something is screwed up somewhere.

And, by the way, if you're getting corrupted data on a server crash, and the server is linux, you just had an object lesson on why it's bad that linux NFS defaults to async writes. :)

Try rwall or similar

[ctr2sprt]

There's a network-able version of wall that uses RPC (I think). It's not a foolproof solution, since it won't work if your users are logged in without an open terminal window, but it's a help. I'm sure it's terrifically insecure, but since you're running NFS you're already insecure (and so hopefully have a firewall).

If that isn't good enough for you, there are a couple other possibilities. You could probably cobble together an utterly trivial Python (or Perl or whatever) script on your client machines, then have the server invoke it via ssh when a shutdown starts. If you aren't a programmer at all, you could try firing off an email to the client machines. As long as you have a periodic mail-checker going, it would alert you to the arrival of a new message. (Since you'd be able to use the local spool, you could have it check every 15 seconds.)

Yeah, but the client thinks it's stateful

[bill_mcgonigle]

put a program on each client machine, call it nfsmounts and it would go a little something like this:

mount | grep $1 | wc -l

then write a wrapper on the server that does

foreach client (client list)
mounted = ssh client nfsounts `hostname`
ok = false if (mounted)
end

you can hook that into your shutdown script, and abort if there are any clients who think they have a mounted drive.

of course, read the other suggestions about mount options. Noone's mentioned sync yet, but don't mount your shares async even though the performance is so much better or you'll loose data.