Re: O_DIRECT in freebsd

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > _That_ is an excellent point. However, do we know at the time we open
> > the file descriptor if we will be doing this?
>
> We'd have to say on a per-read basis whether we want O_DIRECT or not,
> and fd.c would need to provide a suitable file descriptor.

OK

> > What about cache
> > coherency problems with other backends not opening with O_DIRECT?
>
> If O_DIRECT introduces cache coherency problems against other processes
> not using O_DIRECT then the whole idea is a nonstarter, but I can't
> imagine any kernel hackers would have been stupid enough to allow that
> to happen ...

Seeing how the buffer exists in user space, I would be interested how
they prevent coherency problems with good performance --- maybe they map
the same page into multiple processes --- that would be interesting,
though it would require some locking.

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