After several years of use, the server for my development machines developed a problem. If I rebooted the machine without an internet connection, it would pause for several minutes while starting the NFS service.
The Linux NFS server keeps track of status in several files. Whenever a client mounts a filesystem, the server records this in /var/lib/nfs/rmtab. If the server reboots, exportfs passes this list to the kernel to ensure that the reboot is invisible to the clients. So far, this is harmless.
If the server is used with clients that do not send clean unmount requests, such as diskless machines in a development lab, then rmtab fills up with entries for machines that aren’t around anymore. This is mostly harmless.
The final part of the problem is that on reboot exportfs performs a reverse DNS lookup for each entry in rmtab. If the DNS server is unavailable, the request waits for a timeout. This can take a very long time.
And the moral of the story? If NFS clients come and go on your network, check rmtab for clutter.