Path: utzoo!utgpu!water!watmath!clyde!att-cb!att-ih!pacbell!ames!mailrus!umix!uunet!van-bc!sl
From: sl@van-bc.UUCP (pri=-10 Stuart Lynne)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Can IBM-AT disk controller run optimally with no sector interleave?
Keywords: disk,performance
Message-ID: <1686@van-bc.UUCP>
Date: 9 Mar 88 08:23:17 GMT
References: <103@ativax.UUCP>
Reply-To: sl@van-bc.UUCP (Stuart Lynne)
Organization: Public Access Network, Vancouver, BC.
Lines: 32

In article <103@ativax.UUCP> john@ativax.UUCP (John Behrs) writes:
>Although the IBM-AT disk controller runs with a sector interleave of 1,
>it misses consecutive sectors.  Sector interleave of 2 or 3 is optimal,
>even on 16MHZ clones.  Does anyone know the reason for this and if it
>is possible to make an IBM-AT run optimally with no sector interleave?

The standard WD controller only buffers one sector from the disk. When it
successfully reads the sector in (ie, CRC is correct) you (well actually the
disk driver) are notified that you can download it. 

The controller is unable to start reading the next sector until you have
finished empting the sector buffer.

With a 1-1 interleave you only have the inter sector gap time, probably under
50 microseconds. (The disk rotates at 3600 RPM, or 60 cps, giving a rotation 
time of 17 milliseconds, there are typically 17 sectors per track, giving an 
overall time from the start of one sector to the next of 1 millisecond. 
Assume less than 5% overhead for inter-sector gap.)

50 microseconds is not enough time to download much :-) Even a 386 at 20Mhz
will only get part way through.

Western Digital has solved this problem however. Their new controller the
WD-1006 has a buffer capable of holding the entire track, so that it can
continue to read more sectors in while you start taking them out. 

Unfortunately this controller is currently only available sans floppy disk
controller. No word yet on when it will be available with same. It is also
available now with RLL support.

-- 
{ihnp4!alberta!ubc-vision,uunet}!van-bc!Stuart.Lynne Vancouver,BC,604-937-7532