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From: larry@ingres.ARPA (Larry Rowe)
Newsgroups: net.unix-wizards,net.database
Subject: Re: A variant of the streams idea
Message-ID: <172@ingres.ARPA>
Date: Mon, 6-Jan-86 13:06:23 EST
Article-I.D.: ingres.172
Posted: Mon Jan  6 13:06:23 1986
Date-Received: Tue, 7-Jan-86 04:28:33 EST
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Xref: watmath net.unix-wizards:16300 net.database:173

In article <6717@boring.UUCP> jack@mcvax.UUCP (Jack Jansen) writes:
>
>There are great advantages to the file-server model:
>- You don't pay for features you didn't ask for (ever heard
>database people raving about unix readahead?)

In single-user benchmarks read-ahead is a definite win for queries
that scan a lot of pages to answer a query.  The reason should be
pretty obvious: read-ahead overlaps cpu and i/o processing.  a simple
dbms will run very nicely with unix-style read-ahead but a sophisticated
dbms will eventually have to replace the general operating system read-ahead
with a smarter read-ahead.  the reason has to do with what page gets
read on read-ahead.  most dbms's impose a page structure on the data
file that includes a forward pointer to the next primary page and a pointer
to the overflow pages for the current page.  when doing read-ahead,
you want to scan the pages in the order: primary page, overflow page
overflow page, ..., primary page, overflow page, etc. general unix
read-ahead reads the next logically sequential page which won't give
this ordering.
	larry