Re: O_DIRECT in freebsd

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Basically, I think we need free-behind rather than O_DIRECT.
>
> There are two separate issues here --- one is what's happening in our
> own cache, and one is what's happening in the kernel disk cache.
> Implementing our own free-behind code would help in our own cache but
> does nothing for the kernel cache.

Right.

> My thought on this is that for large seqscans we could think about
> doing reads through a file descriptor that's opened with O_DIRECT.
> But writes should never go through O_DIRECT. In some scenarios this
> would mean having two FDs open for the same relation file. This'd
> require moderately extensive changes to the smgr-related APIs, but
> it doesn't seem totally out of the question. I'd kinda like to see
> some experimental evidence that it's worth doing though. Anyone
> care to make a quick-hack prototype and do some measurements?

True, it is a cost/benefit issue. My assumption was that once we have
free-behind in the PostgreSQL shared buffer cache, the kernel cache
issues would be minimal, but I am willing to be found wrong.

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