Is 4.2BSD a failure?

[Henry Spencer]

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