Re: pg_rewind does not rewind diverging timelines

On 7/27/26 01:36, Michael Paquier wrote:
> On Mon, Jun 01, 2026 at 03:30:58PM +0900, Kyotaro Horiguchi wrote:
>> I wonder whether strengthening the history-based matching would be
>> sufficient instead. If timelines with the same TLI but different
>> histories can be treated as distinct and pg_rewind continues walking
>> the history chain until it finds a common ancestor, that seems like a
>> fairly natural fit with the existing timeline model.
>
> Yeah, perhaps there is something that we could do here better in
> pg_rewind in terms of TLI history. A second software layer to ensure
> TLI unicity feels just like a shortcut: we already have a LSN.

As Ants said, the LSN can easily be the same in both.

>> UUIDs would certainly make identification straightforward, although
>> they would also introduce longer identifiers that are a bit less
>> convenient for humans to work with. My initial thought is that it may
>> be worth exploring how far we can get with the existing history
>> information before introducing a new identifier.
>
> This. For this reason, I am not convinced that this proposal is
> neither acceptable or something that we need to do at all.
>
> Why should we bear the burden of introducing a second level of
> timeline identification knowing that by design we have to rely on a
> *single* archive location for a new timeline selection when a standby
> is triggered for promotion? My opinion is that this is trying to fix
> a problem for something that it not actually a problem. If you play
> with HA scenarios where there is a risk of two standbys reusing the
> same timeline number, just don't do that. We've historically claimed
> that the archive strategy is wrong if your deployments cannot
> guarantee a unique TLI assignment. A new sub-identification system
> will not provide more guarantees.

Having had to debug issues with pg_rewind and timelines in our
Kubernetes Operator these UUIDs would probably have helped me a lot.
Maybe the proposed fix is wrong (maybe like Ants says it does not got
far enough) but from personal experience I disagree with that there is
no problem here.

I will try to find time to play around with the patch and see if it
would have helped us.

--
Andreas Karlsson
Percona