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 23:12:44 |
Message-ID: | 200201042312.g04NCir06014@candle.pha.pa.us |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> 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.
I found this before adding the item:
http://www.pairlist.net/pipermail/flow-tools/2001-October/000058.html
And this for FreeBSD 4.4:
2.1 Kernel Changes
The O_DIRECT flag has been added to open(2) and fcntl(2). Specifying this
flag for open files will attempt to minimize the cache effects of reading
and writing.
I also found:
http://www.ukuug.org/events/linux2001/papers/html/AArcangeli-o_direct.html
These later ones seem to indicate there isn't read-ahead, meaning we
would have to do our own prefetches. Eck. I am unclear if that is true
on all OS's.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
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