Bug 771131

Description of problem:

We're running NFS between a bunch of vanilla fedora 16 installations
on x86_64 bare metal machines (no virtualization involved here).

The clients mount their $HOME from the server through an entry in
/etc/fstab like this:

server:/home  /home  nfs4  noatime,rsize=8192,wsize=8192,timeo=14,soft  0 0

Everything works fine and without any problems.  To improve network
performance / throughput we decided to enable support for jumbo
packets by increasing the MTU on all systems from the default 1500 to
9000.  This works fine, too, for most applications.  Network tests show
that we actually use jumbo packets then.

However, NFS stops working.  We see no indication of errors on the
server machine, but the clients report:

[108567.574198] nfs: server ... not responding, timed out
[108571.778790] nfs: server ... not responding, timed out
[108575.983417] nfs: server ... not responding, timed out
[108585.565459] nfs: server ... not responding, timed out
[108589.770074] nfs: server ... not responding, timed out
[108593.974690] nfs: server ... not responding, timed out
[108598.179302] nfs: server ... not responding, timed out
[108602.383908] nfs: server ... not responding, timed out
[108606.588558] nfs: server ... not responding, timed out
[108610.793141] nfs: server ... not responding, timed out
[108614.997763] nfs: server ... not responding, timed out
[108619.202374] nfs: server ... not responding, timed out
[108623.406982] nfs: server ... not responding, timed out

Attempts to "cd $HOME" or "ls" will hang and finally time out with
"I/O errors".

Switching the MTU on the NFS server back to 1500 will make the
problem go away.

Version-Release number of selected component (if applicable):

We experienced the same issue under Fedora 15, but decided to wait
for an software and hardware update and then try again.  Now, under
Fedora 16, we see still the same issue.

Server and all clients are running  kernel-3.1.6-1.fc16.x86_64


How reproducible:

absolutely reliable.

Steps to Reproduce:
1. Set up a NFS4 server / client installation
2. verify that everything is workign fine
3. enable support for jumbo packets by raising the MTU to 9000.
  
Actual results:

NFS I/O errors on the clients, kernel messages "nfs: server ... not
responding, timed out"

Expected results:

No errors, improved performance.

Additional info:

In an older setup, some network cards would not allow a MTU of 9000,
they maxed out at 7200. Running the same test with a MTU of 7200
instead of 9000 still shows the same problem.