A variant of the streams idea

[Larry Rowe]

In article <6717 at boring.UUCP> jack at 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