Re: Corruption during WAL replay

On Wed, 15 Apr 2020 at 04:04, Teja Mupparti <tejeswarm@hotmail.com> wrote:
>
> Thanks Kyotaro and Masahiko for the feedback. I think there is a consensus on the critical-section around truncate,
butI just want to emphasize the need for reversing the order of the dropping the buffers and the truncation.
 
>
>  Repro details (when full page write = off)
>
>          1) Page on disk has empty LP 1, Insert into page LP 1
>          2) checkpoint START (Recovery REDO eventually starts here)
>          3) Delete all rows on the page (page is empty now)
>          4) Autovacuum kicks in and truncates the pages
>                  DropRelFileNodeBuffers - Dirty page NOT written, LP 1 on disk still empty
>          5) Checkpoint completes
>          6) Crash
>          7) smgrtruncate - Not reached (this is where we do the physical truncate)
>
>  Now the crash-recovery starts
>
>          Delete-log-replay (above step-3) reads page with empty LP 1 and the delete fails with PANIC (old page on
diskwith no insert)
 
>

I agree that when replaying the deletion of (3) the page LP 1 is
empty, but does that replay really fail with PANIC? I guess that we
record that page into invalid_page_tab but don't raise a PANIC in this
case.

Regards,

-- 
Masahiko Sawada            http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services