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From: wls@astrovax.UUCP (William L. Sebok)
Newsgroups: net.unix-wizards
Subject: Re: m & n values for disks
Message-ID: <164@astrovax.UUCP>
Date: Sat, 3-Dec-83 14:51:18 EST
Article-I.D.: astrovax.164
Posted: Sat Dec  3 14:51:18 1983
Date-Received: Sun, 4-Dec-83 09:22:28 EST
References: <126@hsi.UUCP>
Organization: Princeton Univ. Astrophysics
Lines: 21

>   - Calculate the next integer greater than
>		disk BPS / CPU BPS
>     and you have the interleave factor, m.
>     As Ed Bryant at Simon Fraser found out by tests, the m value is
>     much more important than the n value.

> In summary, the 4.1 BSD mkfs manual page that says to use m=3 all the time
> is wrong.  Calculate your own values ! (But you don't need to run
> all the timing tests - use the formula above.)

Still one needs to run timing tests to establish the CPU BPS figure, especially
if your system is not one of the types on the above list.  But as said above
once one knows this number for one disk one should be able to compute it for
the other disks on the system.  I believe that 4.2 may do something like this.
  One the other hand, one thing still worries me,  that different disk drivers
may have different disk overheads, especially if they are very different, like,
say, one is a massbus disk and the other a unibus disk.  Anybody have any
comments on that?
-- 
Bill Sebok			Princeton University, Astrophysics
{allegra,akgua,burl,cbosgd,decvax,ihnp4,kpno,princeton}!astrovax!wls