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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.unix-wizards
Subject: Re:  Is 4.2BSD a failure?
Message-ID: <5004@utzoo.UUCP>
Date: Sat, 2-Feb-85 20:20:20 EST
Article-I.D.: utzoo.5004
Posted: Sat Feb  2 20:20:20 1985
Date-Received: Sat, 2-Feb-85 20:20:20 EST
References: <7552@brl-tgr.UUCP>, <158@dmsd.UUCP>
Organization: U of Toronto Zoology
Lines: 16
Keywords: 4.2BSD, file system

Actually, we came up with a rather different idea about why 4.2BSD's
"fast file system" gives such disappointing results in practice.  It
expends quite a bit of effort to try to keep related blocks on the same
cylinder (or nearby).  However, note that related blocks are seldom
accessed simultaneously; there is usually some processing delay in
between.  Keeping related blocks near each other is a win *only* if
the disk heads will probably still be in the vicinity when the next
access occurs.  And... the 4.2BSD filesystem benchmarks were all run
*single-user*!!!  So of course it's nowhere near that good when a
dozen other processes are competing for the disk heads.  The speed
improvements that *are* seen are probably mostly just a result of
bigger blocks.  We conjecture that the "cylinder group" approach
is of little or no benefit for orthodox multi-user time-sharing.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry