Re: O_DIRECT use

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.

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