NFS rename sometimes hangs for 15 seconds after upgrade to Debian 8

[Vincent Lefevre <vincent@vinc17.net>]

At my lab, with NFS accounts, some machines have been upgraded to
Debian 8, and I get regular hangs on these machines, while they
never occurred before on the same machine and still never occur
on a machine that is still under Debian 7.

This happens with one of my scripts, which does a lot of "mv",
and more often when the target has just been modified from another
machine.

A strace showed that it is the "rename" system call that hangs:

52026      0.000208 execve("/bin/mv", ["mv", "-v", "ssh/ssh-reconfigure", "/home/vlefevre/bin/ssh-reconfigu"...], [/* 112 vars */]) = 0
[...]
52026      0.000097 rename("ssh/ssh-reconfigure", "/home/vlefevre/bin/ssh-reconfigure") = 0
52026     15.057725 lseek(0, 0, SEEK_CUR) = -1 ESPIPE (Illegal seek)
52026      0.000189 close(0)            = 0
52026      0.000175 close(1)            = 0
52026      0.000093 munmap(0x7fd5a610b000, 4096) = 0
52026      0.000079 close(2)            = 0
52026      0.000106 exit_group(0)       = ?
52026      0.000307 +++ exited with 0 +++

The NFS server isn't particularly loaded.

Has anyone seen a problem like that?
Where does it come from?
And why does it affect only Debian 8 machines? A new bug?

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)