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From: u-gclapp%ug.utah.edu@cs.utah.edu (Glenn Clapp)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Transfer rates
Message-ID: <1989Dec16.192948.27568@hellgate.utah.edu>
Date: 17 Dec 89 02:29:48 GMT
References: <44807@bu-cs.BU.EDU>
Organization: University of Utah CS Dept
Lines: 30

In article <44807@bu-cs.BU.EDU> ngeow@cs.bu.edu (Yee Ngeow) writes:
>Hi!
>
>  Just a question about hard disk transfer rates. I recently purchased a 
>SCSI drive, the ST-02 (no flames please) and ST-157N. Running with the 
>zero wait jumper enabled, with a 1:1 interleave I am getting 500K of
>transfer rate, but running at 2:1 I am getting 350K or so. I thought 1:1 is
>twice as fast as 2:1, since the disk must rotate twice to read all the sectors.
>Anyone know what is going on there?
>
Well, if your drive has its optimum transfer rate at 2:1 (consecutive
sectors are not really consecutive, but have another sector between them),
then going to a 1:1 interleave will cause the drive to make a full EXTRA
revolution+ to read the next sector, because the drive is too slow to
be ready for it the first time it comes by.  This will DRASTICALLY
reduce your transfer rate.  Going to a higher interleave such as 3:1 will
only slightly reduce transfer rate as the drive will be waiting for data.

Great utilities exist for checking and changing interleave such as 
Gibson Research's SpinRite II, but I don't think it works with SCSI
drives.

In any case, transfer over the SCSI may be very high, but it still has
to get the data into the PC via the out-dated AT bus at 8MHz (some 
are faster, but this get's dicey (I know, just try an ATI VGA Wonder
in a 12MHz bus and see what happens).  The AT bus has a maximum
*Theoretical* transfer rate of 1Mb/s, but in practice the maximum is
somewhat less.

Glenn