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From: mat@amdahl.UUCP (Mike Taylor)
Newsgroups: comp.periphs,comp.unix.wizards
Subject: Re: Fujitsu 2361's, SC7003's, 4.3BSD
Message-ID: <4956@amdahl.UUCP>
Date: Sun, 4-Jan-87 21:53:52 EST
Article-I.D.: amdahl.4956
Posted: Sun Jan  4 21:53:52 1987
Date-Received: Mon, 5-Jan-87 01:37:01 EST
References: <6387@allegra.UUCP> <1305@cit-vax.Caltech.Edu> <4892@mimsy.UUCP> <1421@cit-vax.Caltech.Edu>
Organization: Amdahl Corp, Sunnyvale CA
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Xref: mnetor comp.periphs:95 comp.unix.wizards:496

In article <1421@cit-vax.Caltech.Edu>, mangler@cit-vax.Caltech.Edu (System Mangler) writes:
> 
> The object of sector searching is to avoid tying up the controller
> with disk rotational latency.  It only gains something if another
> drive is ready to do a transfer immediately.  This is an unlikely
> event on a low-end VAX, which doesn't have enough CPU power to
> keep a Massbus controller that busy, and which can ill afford the
> additional interrupts.
> 

Sector searching was introduced (as far as I know - there may have been 
prior art) by IBM with the 3330 DASD in about 1970, called Rotational
Position Sensing.  The idea was to optimize the use of S/370 channels,
rather than control units. It worked OK, but now has been enhanced to
allow the reconnecting controller to select among multiple attached channels,
thus improving the chances of a free channel being found when needed.
Since multiple controllers will normally be attached to a channel, the
controller is generally not as scarce a resource.

-- 
Mike Taylor                        ...!{ihnp4,hplabs,amd,sun}!amdahl!mat

[ This may not reflect my opinion, let alone anyone else's.  ]