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help / color / mirror / Atom feedFrom: Vitaly Davydov <[email protected]>
To: Amit Kapila <[email protected]>
Cc: Alexander Korotkov <[email protected]>
Cc: pgsql-hackers <[email protected]>
Cc: Masahiko Sawada <[email protected]>
Cc: [email protected]
Subject: Re: Slot's restart_lsn may point to removed WAL segment after hard restart unexpectedly
Date: Mon, 26 May 2025 18:34:07 +0300
Message-ID: <1c372-68348a00-11-62e0b680@177489907> (raw)
In-Reply-To: <CAA4eK1LmVoZ8jm5Jt3DERN9Z=y6L6xj5O0asw0smWw31Y8KfVw@mail.gmail.com>
References: <1d12d2-67235980-35-19a406a0@63439497>
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<CAA4eK1LmVoZ8jm5Jt3DERN9Z=y6L6xj5O0asw0smWw31Y8KfVw@mail.gmail.com>
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_lsn is updated is a waste of effort after your patch,
> no? If so, why are we okay with that?
I agree, that saving logical slots at advance is a possible waste of effort. But
I don't understand original ideas behind it. I haven't touched it to make
the minimal patch which should not break the existing functionality.
We trim WAL in checkpoint (or restart point) operation only. The slots'
restart_lsn is used to keep the wal from truncation. I believe, we need to
compute the slots' oldest lsn as the minimal value of restart_lsn values only
when executing checkpoint (or restart point). I guess, it doesn't depend 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 xmin values
are a different story than restart_lsn. It is used to avoid tuple deletions by
vacuum and it is updated by a different way. I can't say that
LogicalConfirmReceivedLocation is the right place to update saved on disk 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_lsn. We may investigate and improve it in an another patch.
With best regards,
Vitaly
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Subject: Re: Slot's restart_lsn may point to removed WAL segment after hard restart unexpectedly
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