Re: crash recovery vs partially written WAL

Greetings,

* Bruce Momjian (bruce(at)momjian(dot)us) wrote:
> On Wed, Dec 30, 2020 at 12:52:46PM -0800, Andres Freund wrote:
> > A question from a colleague made me wonder if there are scenarios where
> > two subsequent crashes could lead to wrong WAL to be applied.
> >
> > Imagine the following scenario
> > [ xlog page 1 ][ xlog page 2 ][ xlog page 3 ][ xlog page 4 ]
> > ^flush ^write ^insert
> >
> > if the machine crashes in this moment, we could end up with a situation
> > where page 1, 3, 4 made it out out to disk, but page 2 wasn't.
>
> I don't see any flaw in your logic. Seems we have to zero out all
> future WAL files, not just to the end of the current one, or at least
> clear xlp_pageaddr on each future page.

I've wondered before if we should be doing a timeline switch at the end
of crash recovery...

Thanks,

Stephen