Subject: [nfsv4] more re: more on server cache

From: [email-masked]
Date: Wed, 19 Nov 2003 10:54:08 -0500 (EST)

[good stuff snipped]
> I disagree 100% (or at least 99.9% :-).  The problem with the argument that
> the retry case is only .1% is that if you accept the request (and don't return
> NFS4ERR_RESOURCE), you are saying you can handle the request.  If you have a
> reply cache, and we all do, part of processing the request is dealing with
> retries.  So you are encouraging the client to do something that will cause
> him to come to grief.  True, it is only a small fraction of the times, but if
> you accept a request of that form, the client will do lots of them, so that
> .1% (or whatever it is) will be a non-null set of instances in which you
> will do a retry and get something that is not correct.  It is better to tell
> the client up-front that you can't do the operation.  If there were some way
> to tell the client that you would do the request once, but if he were 
> subsequently to do a retry, it would fail, then he could make a judgment 
> and decide what to do.  But there isn't.  The only thing you can do is to
> promise him that everything will be processed correctly (with your fingers
> crossed) or be honest and say you can't do it.   

Yep, I see your point. I was thinking of a global high water mark instead
of a rule applied per request. (I'm still not sure my server should be
"disciplining" clients, but I'll think about it. Maybe logging "client X is
doing outrageous compound RPCs", so at least the server admin is aware of it.)

When I think about a global high water mark, I was concerned about a large
number of smaller compound RPCs with non-idempotent Op(s) in them flooding
the cache. I am hoping that there is enough feedback through the TCP
connection when the server is under heavy load that that will set a
reasonable upper bound on cache memory requirements, but it remains to be
seen. If the high water mark is set way up at the "one hundred year flood
level", then NFS4ERR_RESOURCE does seem to make sense.

> I would say "just can't continue correctly".

Well put. (I have the traditional NFS mindset that says "Nothing always works
correctly anyway, so 99.9% sounds good", but I think I would agree it is
about time to change that outlook to at least aim for 100%:-)

rick, who still gets nervous when none of the client writers jump into
	these discussions

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