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Re: O_DIRECT use

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: O_DIRECT use
Date: 2002-01-04 21:31:40
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Lists: pgsql-hackers
Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > I have added this item to TODO:
> > 	* Consider use of open/fctl(O_DIRECT) to minimize OS caching
> Why exactly would we wish to minimize OS caching?
> In my mind, Postgres has always relied heavily on the existence of a
> layer of kernel caching.  Disabling that will hurt far more than help.

Not sure. Someone on IRC brought it up.  If we are sequential scanning a
large table, caching may be bad because we are pushing out stuff already
in the cache that may be useful.  It is related to this TODO item:

	* Add free-behind capability for large sequential scans (Bruce)

  Bruce Momjian                        |
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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