Re: crash recovery vs partially written WAL

On Wed, Dec 30, 2020 at 12:52:46PM -0800, Andres Freund wrote:
> Hi,
> 
> 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.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

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