Need an equation to work out file cacheing
From: Gareth Williams (gareth_at_nospam.com)
Date: 08/01/04
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Date: Sun, 1 Aug 2004 19:06:28 +0000 (UTC)
Scenario:
1. A firm receives 100,000 orders per week as electronic documents.
2. All documents are archived on CDROM.
3. A percentage of orders (average around 10%) will go wrong at some
stage and access to the original order will be required by Customer
Services to resolve the problem.
4. Retrieval from CDROM is time-consuming but a limited amount of space is
available to cache some of the documents on networked storage for more
immediate retrieval. There is not enough space to cache all the documents
so the older cached documents are deleted regularly.
5. It is not possible to determine "up front" which orders will fail -
whilst the failure rate is fairly constant over time, just about any
document could end up being requested by Customer Services.
Problem:
The firm would like to cache "N" documents (from the weekly pool of
100,000 orders) such that they can satisfy "X" percent of Customer Service
requests from the networked storage. Customer Services are prepared to
put up with [100-X]% of requests that would still need to be retrieved
from the CDROM store.
In short, how do we calculate "N"?
This reminds me a little of the "Cookie Jar" or "Sock Drawer" problem,
but it bugs me that I can't puzzle it out. I'm not a hard-core
statistician and would be grateful for any help. This is a genuine (i.e.
non-homework) request, by the way.
-- Regards, Gareth Williams
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