Horton

Re: pg_rewind WAL segments deletion pitfall

Alexander Kukushkin <cyberdemn@gmail.com>

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From: Alexander Kukushkin <cyberdemn@gmail.com>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: bungina@gmail.com, pgsql-hackers@lists.postgresql.org
Date: 2022-08-30T08:03:07Z
Lists: pgsql-bugs, pgsql-hackers 
Hello Kyotaro,


On Tue, 30 Aug 2022 at 09:51, Kyotaro Horiguchi <horikyota.ntt@gmail.com>
wrote:

>
>
> And as I said in a mail I sent just before, the patch looks too
> complex.  How about just comparing WAL file name aginst the last
> common checkpoint's tli and lsn? We can tell filemap.c about the last
> checkpoint and decide_file_action can compare the file name with it.
>
> It is sufficient to preserve WAL files if tli matches and the segment
> number of the WAL file is equal to or later than the checkpoint
> location.
>

What if the last common checkpoint was on a previous timeline?
I.e., standby was promoted to primary, the timeline changed from 1 to 2,
and after that the node crashed _before_ the CHECKPOINT after promote has
finished.
The next node will advance the timeline from 2 to 3.
In this case, the last common checkpoint will be on timeline 1, and the
check becomes more complex because we will have to consider both timelines,
1 and 2.

Also, we need to take into account the divergency LSN. Files after it are
not required.

Regards,
--
Alexander Kukushkin

Commits

  1. Fix newly introduced 010_keep_recycled_wals.pl

  2. Avoid deleting critical WAL segments during pg_rewind