Re: nfs udp 1000/100baseT issue

[Helge Hafting]

Bret Towe wrote:

> On 3/16/06, Neil Brown <neilb@xxxxxxx> wrote:
>  
>
> > There is no flow control in UDP
> >    
>
> is this a linux design flaw or just nature of udp?
>  
That has nothing to do with linux at all.

"Now flow control in udp" is a udp design issue.  And it is not
a flaw either - the rule is simple:

If you need flow control - use tcp.
If you don't need flow control, and don't want the
overhead of flow control - use udp.

Udp is for those cases where flow control is consideres a waste of time.

Now, the original decision to base early NFS on udp, that was
a design mistake.  Again, not a linux problem but a nfs problem.
Fortunately, today a solution for this exists and is implemented
in linux - and it is nfs over tcp.

> > .  If anything gets lots, the client
> > has to resend the request, and the server then has to respond again.
> > If the respond is large (e.g. a read) and gets fragmented (if > 1500bytes)
> > then there is a good chance that one or more fragments of a reply will
> > get lots in the switch stepping down from 1G to 100M.  Every time.
> >
> > Your options include:
> >
> >  - use tcp
> >    
>
> im wondering why this isnt the default to begin with
>  
Hard to say.  I guess someone thought they could get better
performance with udp - it has less overhead.,
Then didn't bother testing this idea with a somewhat congested network?

Helge Hafting
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