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From: enchant@oliveb.UUCP (Dan Crocker)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Interleave factor on a 6300
Message-ID: <3002@oliveb.UUCP>
Date: Tue, 11-Aug-87 16:46:06 EDT
Article-I.D.: oliveb.3002
Posted: Tue Aug 11 16:46:06 1987
Date-Received: Thu, 13-Aug-87 07:04:48 EDT
References: <2928@mtgzz.UUCP> <1844@ttrdc.UUCP> <300@cpsc6b.cpsc6a.att.com>
Organization: Olivetti ATC; Cupertino, Ca
Lines: 40

> No.  The interleave values for a hard disk are based on the performance of
> the disk, not the system.  It basically determines the number of cylinders
> the head must pass after reading from/writing to a cylinder before it can
> read from/write to another cylinder.  If the drive takes 4 cylinders to
> reset, and you set the interleave to 3, the head will wait for almost a
> complete revolution before attempting another read/write.  Setting the value
> to 5 would mean that the drive is basically idle for one cylinder pass.

I suspect you meant sector and not cylinder in the above explanation.
To clarify, the interleave factor is dependant on both the system and 
controller performance.  A proper interleave will allow enough time for 
a sector to be read from the disk and its data to be transferred to the
system BEFORE the next logical sector passes by the head.  The time it takes
to read the data from the disk is solely dependant on the rotation speed of
the disk.  The interleave allows for: delay of controller in notifying system
of data available, time required by system to transfer data, and time required
by controller to become ready to look for the next sector or receive a new
command.  Therefore, moving the same disk/controller combination from an 
8088 based system to a 386 based system could have a drastic effect on the
allowable interleave.  Obviously, other system performance factors can have
a similar impact.

Two additional points: 

-The required interleave for single-sector accesses can be greater than that
 for multi-sector accesses.  It depends on how much time is taken between
 sector requests.

-Obviously, interleave is only important when you are reading logically
 consecutive sectors.  In other words, if most of your files on your disk 
 are fragmented, then changing the interleave will not show such a drastic 
 improvment.


				Dan
-- 
Here comes the supernatural anesthetist
If he wants you to snuff it
All he has to do is puff it
He's such a fine dancer