Re: O_DIRECT use - Mailing list pgsql-hackers

From Bruce Momjian
Subject Re: O_DIRECT use
Date
Msg-id 200201042131.g04LVes26851@candle.pha.pa.us
Whole thread Raw
In response to O_DIRECT use  (Bruce Momjian <pgman@candle.pha.pa.us>)
Responses Re: O_DIRECT use  (Brent Verner <brent@rcfile.org>)
Re: O_DIRECT use  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-hackers
Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.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                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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