Not Fully Serviceable 285By design, NFS is connectionless and stateless. In practice, it is neither.
This conflict between design and implementation is at the root of most
NFS problems.
“Connectionless” means that the server program does not keep connections
for each client. Instead, NFS uses the Internet UDP protocol to transmit
information between the client and the server. People who know about net-
work protocols realize that the initials UDP stand for “Unreliable Data-
gram Protocol.” That’s because UDP doesn’t guarantee that your packets
will get delivered. But no matter: if an answer to a request isn’t received,
the NFS client simply waits for a few milliseconds and then resends its
request.
“Stateless” means that all of the information that the client needs to mount
a remote file system is kept on the client, instead of having additional infor-
mation stored on the server. Once a magic cookie is issued for a file, that
file handle will remain good even if the server is shut down and rebooted,
as long as the file continues to exist and no major changes are made to the
configuration of the server.
Sun would have us believe that the advantage of a connectionless, stateless
system is that clients can continue using a network file server even if that
server crashes and restarts because there is no connection that must be rees-
tablished, and all of the state information associated with the remote mount
is kept on the client. In fact, this was only an advantage for Sun’s engi-
neers, who didn’t have to write additional code to handle server and client
crashes and restarts gracefully. That was important in Sun’s early days,
when both kinds of crashes were frequent occurrences.
There’s only one problem with a connectionless, stateless system: it
doesn’t work. File systems, by their very nature, have state. You can only
delete a file once, and then it’s gone. That’s why, if you look inside the
NFS code, you’ll see lots of hacks and kludges—all designed to impose
state on a stateless protocol.
Broken Cookie
Over the years, Sun has discovered many cases in which the NFS breaks
down. Rather than fundamentally redesign NFS, all Sun has done is hacked
upon it.
Let’s see how the NFS model breaks down in some common cases: