Re: O_DIRECT use

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Tom Lane wrote:
> >> Why exactly would we wish to minimize OS caching?
>
> > 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.
>
> Yeah, but people normally try to set things up to avoid doing large
> sequential scans, at least in all the contexts where they need high
> performance. For index searches you definitely want all the caching
> you can get.
>
> 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.

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