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| Reply To: | [log in to unmask][log in to unmask], 5 Sep 1997 09:19:27 -0500512_us-ascii A few months ago, I helped a customer connect to the Internet. We called in an ISP and they gave us a range of IP addresses. We modified the network to use those addresses, then we waited for SWB (South Western Bell) to install the IDSN (128kbps), ten days. When the ISDN was installed, the ISP came in with an Ascend box, placed it on the internal network, connected the ISDN line to it, instructed the built-in firewall about a few things, and bingo, they were on the Internet. [...]37_5Sep199709:19: [log in to unmask] |
| Date: | Fri, 5 Sep 1997 10:51:52 -0700 |
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Image will allow you to begin at end-of-chain and read the chain in
reverse (DBGET-mode 6, I think). The inefficiency referred to is
probably due to the way records are retrieved from disk. Image
retrieves blocks of data in increments of 4096 bytes (2048 half-words).
Assuming the chain you're reading is optimally packed - i.e., all
blocks contiguous - the prefetch characteristics cause the blocks to be
read forward and will read 1-to-n contiguous entries of the chain into
memory, reducing the number of I/O's required to retrieve all the
records. A reverse chained read doesn't cause the blocks to be read
"backward"; therefore, this will usually incur more disk I/O's to
retrieve all the chain entries.
Typically, a reverse chained read is most useful for applications
needing data which are typically stored in a sequence for which the
most recently added entry/entries are required.
I hope this helps (and is essentially accurate :-).
Lee Gunter
Regence Blue Cross Blue Shield of Oregon / Regence HMO Oregon
mailto:[log in to unmask]
voice...503-375-4498
fax.....503-375-4401
==========================================================
The opinions expressed, here, are mine and mine alone, and do not
necessarily reflect those of my employer.
______________________________ Reply Separator _________________________________
Subject: Re: [HP3000-L] Image Inefficient?
Author: "Michael A. Dobies" <[log in to unmask]> at ~INTERNET
Date: 9/5/97 8:35 AM
Don't you have to go to the endof chain, before you can go backwards?
Michael Dobies
-----Original Message-----
From: Gary Groves [SMTP:[log in to unmask]]
Sent: Friday, September 05, 1997 7:49 AM
To: [log in to unmask]
Subject: Image Inefficient?
I need to create a new Image dataset. I want to retrieve the records in
reverse date order (LIFO). I was going to do a backwards chain read.
Someone told me that a reverse chain read was not efficient.
For other design reasons, I've decided to use a composite Omnidex key,
but I'm curious if the above is true. If so, Why?
Gary Groves
Kwestions, Kwalms, Kweries, Komments....Kall!
http://www.superstar.com
http://www.netlinksat.com
http://www.uvsg.com
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