From: "Vitaly Davydov" <v.davydov@postgrespro.ru>
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Date: Mon, 26 May 2025 18:34:07 +0300
Cc: "Alexander Korotkov" <aekorotkov@gmail.com>,
 "pgsql-hackers" <pgsql-hackers@postgresql.org>,
 "Masahiko Sawada" <sawada.mshk@gmail.com>, tomas@vondra.me
To: "Amit Kapila" <amit.kapila16@gmail.com>
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Message-ID: <1c372-68348a00-11-62e0b680@177489907>
Subject: =?utf-8?q?Re=3A?= Slot's =?utf-8?q?restart=5Flsn?= may point to
 removed WAL
 segment after hard restart unexpectedly
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Dear Amit,

> OTOH, if we don't want to adjust physical
> slot machinery, it seems saving the logical slots to disk immediately
> when its restart=5Flsn is updated is a waste of effort after your pat=
ch,
> no? If so, why are we okay with that?

I agree, that saving logical slots at advance is a possible waste of ef=
fort. But
I don't understand original ideas behind it. I haven't touched it to ma=
ke
the minimal patch which should not break the existing functionality.

We trim WAL in checkpoint (or restart point) operation only. The slots'
restart=5Flsn is used to keep the wal from truncation. I believe, we ne=
ed to
compute the slots' oldest lsn as the minimal value of restart=5Flsn val=
ues only
when executing checkpoint (or restart point). I guess, it doesn't depen=
d on
slot's type (logical or physical). We have 0003 patch to fix it.

I haven't deeply investigated yet slot's xmin values but I guess the xm=
in values
are a different story than restart=5Flsn. It is used to avoid tuple del=
etions by
vacuum and it is updated by a different way. I can't say that
LogicalConfirmReceivedLocation is the right place to update saved on di=
sk xmin
values. I would propose to update these values in SaveSlotToPath under =
some lock
to avoid concurrent reads of unsaved values or do in a checkpoint like =
as for
restart=5Flsn. We may investigate and improve it in an another patch.

With best regards,
Vitaly