Re: NFS alternative

[Brian May <bam@debian.org>]

>>>>> "D-Man" == D-Man  <dsh8290@rit.edu> writes:

    D-Man> Interesting ... from my understanding UDP is a
    D-Man> connectionless protocol, and as such packets aren't
    D-Man> guaranteed to arrive at the destination.  It seems to me
    D-Man> that for a file system, you really want all the packets to
    D-Man> arrive.  How is this not a problem?  (BTW, I'm just

Well, in theory, all UDP packets could be numbered, much like TCP
packets, and you get exactly the same reliability TCP offers. This
effectively means you are tunnelling a connection based protocol over
a connectionless based protocol (in much the same way TCP uses
IP). You might be able to do this with less overhead then TCP, but
personally I would just use TCP, as that way, other people are
responsible for the tedious tasks of congestion control, etc).

I have heard though, that NFS uses RPC, and RPC doesn't do anything
this sophisticated. Rather, if the server doesn't respond in time, the
client sends the request again.

This is OK for requests like "read file", which don't affect the state
of the server, as the worst thing that could happen is that the server
will reread the file again.

For other requests though, like "delete file", confusion could
occur. eg. consider the case where the client wants to delete file X,
and the file gets deleted, but the response is lost. The client
resends the request again, and the server processes that request
again, but this time says "file not found" because it no longer exists
(it has no way of knowing that this was the same request it just
received). The client gets totally confused, because it as far as it
was concerned, the file did exist.

Could be more serious, I imagine for "append file" operations, where
you could get the data appended twice...

I once read a book that said care needs to be taken with any RPC call
that alters the state of the server, for this reason, but from what I
have heard, NFS doesn't care.

Conclusion: (if my sources are correct) never use NFS (unless mounted
read-only) over a congested network...
-- 
Brian May <bam@debian.org>