Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!mks!wheels
From: wheels@mks.UUCP
Newsgroups: comp.sys.atari.st
Subject: Re: Disk speeds on Atari ST and Macintosh
Message-ID: <211@mks.UUCP>
Date: Mon, 2-Feb-87 10:26:04 EST
Article-I.D.: mks.211
Posted: Mon Feb  2 10:26:04 1987
Date-Received: Tue, 3-Feb-87 01:41:57 EST
References: <8701290450.AA16048@cory.Berkeley.EDU> <2165@batcomputer.tn.cornell.edu>
Organization: Mortice Kern Systems, Waterloo, Ont.
Lines: 38
Summary: what about interleaving?



During this discussion about disk transfer rates, I have not seen
anyone mention the effects of the interleaf (interleave?) factor.

By the way, Atari, what is the interleaving factor for the standard
formatter?

If, for example, the factor was two, then logical sector #2 would be
two sectors beyond logical sector #1. While the intervening sector
is passing under the heads, the OS or application can process the
data from sector #1 and get ready for the next read.

Some programs take more time than others to process what they have
just read. If they take too long, they will miss the next logical
sector and will have to wait one whole revolution to get it.

If one knows in advance how a particular disk will be used, one can
tweak the interleave to suit. For example, my original copy of
ST Raider loads more quickly than my backup copy. You can actually
hear the difference in the head stepping rate. I think the original
must have different interleaving, tuned to the loading of programs
into memory (little processing needed).

The standard formatter must strike a compromise. If sequential logical
sectors are too close, some programs will read (or write) more slowly
than necessary. If the sectors are spread too far apart, fast programs
are penalised and have to wait for the sector to come around.

In my experience on various microcomputers, this one factor has had more
effect on disk transfer rates than any other. Some formatters give a
choice of interleave factor for hand-tweaking.

By the way, if you're about to format a hard disk, and if you have the
option, try different interleave factors before putting too much data
on the disk.

     Gerry