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From: hedrick@athos.rutgers.edu (Charles Hedrick)
Newsgroups: comp.dcom.lans,comp.unix.wizards
Subject: Re: NFS performance: a question
Message-ID: <705@athos.rutgers.edu>
Date: 1 Feb 88 03:26:39 GMT
References: <663@noao.UUCP>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 16
Keywords: NFS, performance, read/write asymmetry
To: brown@noao.arizona.edu


You note that reading is faster than writing via NFS.  We see this on
Suns also, though the difference we see is not quite as drastic as
yours.  Some tests I just did showed writing a big file to be about a
factor of 3 slower than reading.  I believe this is because reading
can do readahead, but it is hard to predict what the next byte a user
is going to write is going to be.  (Note that the machine doing the
reading has a Ciprico controller, so the controller is doing a lot of
readahead also.  Reading across the network was actually twice as fast
as reading locally, because the local machine had a normal Xylogics
controller.)  There are some other things that can affect NFS
performance though.  try nfsstat before and after your test.  See if
you are getting retransmissions.  We have run into systems that can
send data faster than they can receive them (Sun 4's).  In that case,
it turned out that not running biod actually improved throughput by a
factor of 3.  For more normal cases, biod does help.  You can
sometimes increase performancy by running more copies, e.g. /etc/biod
8 instead of /etc/biod 4.