Re: NFS alternative

[D-Man <dsh8290@rit.edu>]

On Mon, Jul 16, 2001 at 12:13:10PM -0700, Francois Gouget wrote:
| On Mon, 16 Jul 2001, D-Man wrote:
| > On Mon, Jul 16, 2001 at 12:45:22PM -0500, Dimitri Maziuk wrote:
| > | * D-Man (dsh8290@rit.edu) spake thusly:
| > | ...
| > | > Ok, that makes sense.  How about if probability leaves us behind
| > | > and a packet is lost?  Does NFS provide any way to correct for
| > | > that or will your filesystem be hosed?
| > | 
| > | Thankfully, I forget the details[0]. From experience, no, it won't
| > | be exactly hosed: you'll end up with a .nfs004950384672385721380937
| > | file that will grow and eventually fill up the partition... nothing
| > | an rm -rf / won't fix. And then there's negative cookies and stale
| > | mounts that require a reboot on most unices I've seen... 
| > 
| > Ok.  It sounds like it would still result in data loss :-(.
| 
|    Hmm, I'm not an NFS expert but I'll play one on the mailing-list for
| you ;-) Please, if there are experts out there, correct me if I'm wrong.

<grin>  Everyone's an expert on the internet :-).

|    AFAIU, NFS has its own mechanism to recover from lost packets. So it
| won't be a problem if a packet is lost. Similarly I believe NFS RPCs

That's nice to know.

| cannot span UDP packets, so there is no chance that a lost packet would
| change the meaning of an RPC. The RPC will be lost, pure and simple, and
| NFS will have to reissue it or something similar. So I don't think
| packet loss is an issue.

Good.

|    What NFS is 'lacking' is congestion control, as in the TCP slow star
| and exponential back-off. This means NFS will blast UDP packets as fast
| as it cans with no regard for other trafic. This is not really an issue
| on a lan and actually had a performance at a time (I think). But if you
| go over multiple links, then you may saturate a slower link, causing the
| router that is just before it to start dropping packets. Especially if
| multiple streams converge there. And once you start dropping packets
| performance degrades very significantly. I believe that's why NFS is bad
| if there are multiple hops (I get it from a very reliable source that
| this is also why it's very bad if the traffic will go over ATM, you need
| buffering/traffic shaping).

Ok, yeah.  I'm not worried about congestion on my home LAN, I was just
curios about (theoretical) reliability knowing it used UDP.

Thanks for that info!

-D