Itkovian.net

People at our department are often running jobs that take multiple days to complete. A good way to keep a job going is to use the screen tool. This allows one to keep running a shell even when not connected to the machine for a while, and to reconnect to that session when you login on the machine once more. Simple, lean and mean.

Suppose you wish to run a job, and save its output to a file that resides on your home directory, which is, incidentally, and AFS mount. You log in, get a token (as per the manual method described here, fire up screen, start your job, leave screen (using the nice ctrl-a-d keystroke), and log out because you like to take your laptop home (which sounds almost like the purpose of having a laptop in the first place). The next day you arrive at the office, turn on the laptop, connect it to the omnipresent ethernet, and log into the machine (yes, you request a token again) and you check the output of you job ... Not a good idea if you have a faint heart, or that data was really, really due, like yesterday.
Why, the innocent reader now asks? Well, clearly, because the output has not made its way to the file as you would have liked it to do. To cut short the story: the moment you logged out, your AFS token was revokes, or became invalid, or whatever, and the file that was greedily accepting your bits suddenly was inaccessible to your process. For example,

#!/bin/bash

i=0;
while(true); do
echo "hup $i"
sleep 1;
echo "pup $i";
i=`expr $i + 1`;
done;

woud yield this:

hup 0
pup 0
hup 1
pup 1
hup 2
pup 2
hup 3
pup 3
hup 4
hup 17
pup 17
hup 18
pup 18
hup 19
pup 19
hup 20
pup 20
hup 21
pup 21
hup 22

if you had logged out after 4 seconds and logged back in (and acquired a token) about 13 seconds later.

Boooooo! Eat that.

Of course, a simple workaround exists: save the data temporarily to a local file on the machine, e.g., in /tmp (and pray that the tmp dir is not wiped nightly). But that does kind of defeat the purpose of having a secure filesystem on which to store your data, does it not?