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Date: Fri, 18 Aug 2023 15:40:57 +0900
From: torikoshia <torikoshia@oss.nttdata.com>
To: Pgsql Hackers <pgsql-hackers@postgresql.org>
Cc: bungina@gmail.com, cyberdemn@gmail.com,
 pgsql-hackers@lists.postgresql.org, Kyotaro Horiguchi
 <horikyota.ntt@gmail.com>
Subject: Re: pg_rewind WAL segments deletion pitfall
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On 2022-09-29 17:18, Polina Bungina wrote:
> I agree with your suggestions, so here is the updated version of
> patch. Hope I haven't missed anything.

Thanks for the patch, I've marked this as ready-for-committer.

BTW, this issue can be considered a bug, right?
I think it would be appropriate to provide backpatch.

On 2023-06-29 18:42, torikoshia wrote:
> On 2023-06-29 10:25, Kyotaro Horiguchi wrote:
> Thanks for the comment!
> 
>> At Wed, 28 Jun 2023 22:28:13 +0900, torikoshia
>> <torikoshia@oss.nttdata.com> wrote in
>>> 
>>> On 2022-09-29 17:18, Polina Bungina wrote:
>>> > I agree with your suggestions, so here is the updated version of
>>> > patch. Hope I haven't missed anything.
>>> > Regards,
>>> > Polina Bungina
>>> 
>>> Thanks for working on this!
>>> It seems like we are also facing the same issue.
>> 
>> Thanks for looking this.
>> 
>>> I tested the v3 patch under our condition, old primary has succeeded
>>> to become new standby.
>>> 
>>> 
>>> BTW when I used pg_rewind-removes-wal-segments-reproduce.sh attached
>>> in [1], old primary also failed to become standby:
>>> 
>>>   FATAL: could not receive data from WAL stream: ERROR: requested WAL
>>>   segment 000000020000000000000007 has already been removed
>>> 
>>> However, I think this is not a problem: just adding restore_command
>>> like below fixed the situation.
>>> 
>>>   echo "restore_command = '/bin/cp `pwd`/newarch/%f %p'" >>
>>>   oldprim/postgresql.conf
>> 
>> I thought on the same line at first, but that's not the point
>> here.
> 
> Yes. I don't think adding restore_command solves the problem and
> modification to prevent deleting necessary WAL like proposed
> patch is necessary.
> 
> I added restore_command since
> pg_rewind-removes-wal-segments-reproduce.sh failed to catch up
> even after applying v3 patch and prevent pg_rewind from delete
> WALs(*), because some necessary WALs were archived.
> 
> It's not a problem we are discussing here, but I wanted to get
> the script to work to the point where old primary could
> successfully catch up to new primary.
> 
> (*)Specifically, running the script without apply the patch,
> recovery failed because 000000010000000000000003 which has
> already been removed. This file was deleted by pg_rewind as
> we know.
> OTHO without the restore_command, recovery failed because
> 000000020000000000000007 has already been removed even after
> applying the patch.
> 
>> The problem we want ot address is that pg_rewind ultimately
>> removes certain crucial WAL files required for the new primary to
>> start, despite them being present previously.
> 
> I thought it's not "new primary", but "old primary".
> 
>> In other words, that
>> restore_command works, but it only undoes what pg_rewind wrongly did,
>> resulting in unnecessary consupmtion of I/O and/or network bandwidth
>> that essentially serves no purpose.
> 
> As far as I tested using the script and the situation we are facing,
> after promoting newprim necessary WAL(000000010000000000000003..) were
> not available and just adding restore_command did not solve the 
> problem.
> 
>> pg_rewind already has a feature that determines how each file should
>> be handled, but it is currently making wrong dicisions for WAL
>> files. The goal here is to rectify this behavior and ensure that
>> pg_rewind makes the right decisions.
> 
> +1

-- 
Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION