Re: crash recovery vs partially written WAL

On Thu, Dec 31, 2020 at 02:27:44PM -0500, Stephen Frost wrote:
> 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...

For a while we had trouble tracking timeline switches, but I think we
might be fine on that now.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee