Re: Streaming base backups

On Tue, Jan 11, 2011 at 12:45:02PM -0500, Tom Lane wrote:
> Florian Pflug <fgp(at)phlo(dot)org> writes:
> > On Jan11, 2011, at 18:09 , Garick Hamlin wrote:
> >> My gut was that direct io would likely work right on Linux
> >> and Solaris, at least.
>
> > Didn't we discover recently that O_DIRECT fails for ext4 on linux
> > if ordered=data, or something like that?
>
> Quite. Blithe assertions that something like this "should work" aren't
> worth the electrons they're written on.

Indeed. I wasn't making such a claim in case that wasn't clear. I believe,
in fact, there is no single way that will work everywhere. This isn't
needed for correctness of course, it is merely a tweak for performance as
long as the 'not working case' on platform + filesystem X case degrades to
something close to what would have happened if we didn't try. I expected
POSIX_FADV_NOREUSE not to work on Linux, but haven't looked at it recently
and not all systems are Linux so I mentioned it. This was why I thought
direct io might be more realistic.

I did not have a chance to test before I wrote this email so I attempted to
make my uncertainty clear. I _know_ it will not work in some environments,
but I thought it was worth looking at if it worked on more than one sane
common setup, but I can understand if you feel differently about that.

Garick

>
> regards, tom lane