Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!usc!zaphod.mps.ohio-state.edu!magnus.ircc.ohio-state.edu!csn!arrayb!taylor
From: taylor@intellistor.com (Dick Taylor)
Newsgroups: comp.benchmarks
Subject: Re: Measuring disk read times for Unix
Message-ID: <1991Feb27.233049.2972@intellistor.com>
Date: 27 Feb 91 23:30:49 GMT
References: <714@opus.NMSU.Edu>
Distribution: comp
Organization: Intellistor, Longmont, CO
Lines: 26

In article <714@opus.NMSU.Edu> pfeiffer@nmsu.edu (Joe Pfeiffer) writes:
>...
>In order to measure write times, I'm using the O_SYNC to flush my data
>to disk on each write.  So far so good.
>
>I'm having trouble trying to do the equivalent for reads.  I appear to
>be filling up the OS's disk buffers on the first pass through the
>test, after which reads come from there.  While an obvious solution of
>making the test file so huge it won't all fit in there would help, it
>wouldn't be a complete solution.
>
>Any thoughts?

There are a number of ways to do it.  The niftiest that I ever found was
to use the mount() and unmount() system calls to unmount the filesystem
between each iteration of the benchmark.  One warning, though -- this
stresses those two system calls a lot more heavily than anything else
they're used for.  In some versions of SunOS, in particular, there
are interactions with the update daemon that can crash the system
quite spectacularly (this appears to be fixed in SunOS 4.1 and in
Solbourne's OS/SMP-4.0D).  Other ways to do this basically involve flushing
large amounts of data through the filesystem buffers and hoping that
the unwanted data disappears.  Time-consuming and nasty, if you ask me.

			-- Dick Taylor
			   Intellistor, Inc.