[ale] Lab Workstation Mystery

[Todor Fassl]

I have a mysterious problem with workstations in a shared use 
environment. There are 2 labs in different buildings, onewith 6 
workstations and one with 8. These workstations are used by a group of 
about 30 grad student TAs. All are running ubuntu 15.10. Authentication 
is via ldap and home directories are mounted  via nfs.  Every day, 2 or 
3 of the machines go down. The earliest symptom I can find is that the 
root filesystem is remounted read-only.  Soon they stop responding to 
ssh and snmp and they are essentially locked up. They still respond to 
pings though.

I've caught the machines in the period where the root system is 
read-only but I can still ssh to them. I've found that I cannot nfs 
mount home directories on our file server.  I can mount nfs shares on 
other servers. And I can mount the same home directories if I go to 
another workstation. Restarting nfs on the file server has no effect.

When I try to mount a home directory on an effected machine, the mount 
just hangs.  I ran it with strace and it just showed it was waiting -- 
for what, I'm not sure and I don't have a screen cap available at the 
moment. I put a packet sniffer on the server and it showed it received a 
single packet from the client and that's it.

There is nothing in the logs on the client. In fact, they simply stop at 
some point in the process. At first I attributed this to the root 
filesystem being read-only but it continues after I move /var to a 
separate file system. At some point it just stops writing records to the 
syslog but I don't know if it's before or after the root filesystem is 
remounted read-only.

Many of the TAs also have identical workstations in their offices. None 
of those machines seem to have this problem.  The TAs do tend to walk 
away from the workstations w/o logging out. But I wrote a script to kill 
off their sessions and it didn't help. I had it send me an email 
whenever it killed somebody's session and it doesn't seem to be 
correlated with that. In other words, sometimes machines go down even if 
everyone who has used it has remembered to log out.

I'm pretty desperate. Any ideas?