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From: philf@xymox.metaphor.com (Phil Fernandez)
Newsgroups: comp.arch,comp.databases
Subject: Re: *big iron*
Message-ID: <829@metaphor.Metaphor.COM>
Date: 5 Oct 89 19:00:49 GMT
References: <21962@cup.portal.com> <1989Sep12.031453.22947@wolves.uucp> <22130@cup.portal.com> <1989Sep16.044013.429@wolves.uucp> <3752@rtech.rtech.com>
Sender: news@metaphor.Metaphor.COM
Reply-To: philf@xymox.metaphor.com (Phil Fernandez)
Organization: Metaphor Computer Systems, Mountain View, CA
Lines: 39

In article <3752@rtech.rtech.com> daveb@rtech.UUCP (Dave Brower) writes:
> ... Some
>airline reservation systems are said to have huge farms of disk where
>only one or two tracks are used on the whole pack to avoid seeks, for
>instance.

No, I don't think so.

I did a consulting job for United Airlines' Apollo system a couple of
years ago, looking for architectures to break the 1000t/s limit.  We
looked at distributing transactions to many processors and disks,
etc., etc., but nothing quite so profligate at using only a couple of
tracks (or cyls) on a 1GB disk pack in order to minimize seeks.

On the *big iron* that UAL and other reservations systems use, the
operating systems (TPFII and MVS/ESA) implement very sophisticated
disk management algorithms, and in particular, implement elevator
seeking. 

With elevator seeking, disk I/O's in the queue are ordered in such a
way to minimize seek latency between I/O operations.  In an I/O-
intensive tp application with I/O's spread across multiple disk packs,
a good elevator scheduling scheme is all that's needed to get the
appropriate disk I/O bandwidth.

Makes for a good story, tho!

phil





+-----------------------------+----------------------------------------------+
| Phil Fernandez              |             philf@metaphor.com               |
|                             |     ...!{apple|decwrl}!metaphor!philf        |
| Metaphor Computer Systems   |"Does the body rule the mind, or does the mind|
| Mountain View, CA           | rule the body?  I dunno..." - Morrissey      |
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