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Re: O_DIRECT use

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: O_DIRECT use
Date: 2002-01-04 21:57:06
Message-ID: 7244.1010181426@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
>> For that matter, I would expect that O_DIRECT also defeats readahead,
>> so I'd fully expect it to be a loser for seqscans too.

> I am told on FreeBSD it does not disable read-ahead, just caching;
> something that needs more research.

Hmm.  I always thought of read-ahead as preloading buffer cache entries.

It'd be interesting to get a description of *exactly* what this flag
does, rather than handwavy approximations.  Time to start reading the
kernel code, I suppose.

			regards, tom lane

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