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help / color / mirror / Atom feedFrom: Amit Kapila <[email protected]>
To: Alexander Korotkov <[email protected]>
Cc: Vitaly Davydov <[email protected]>
Cc: vignesh C <[email protected]>
Cc: Hayato Kuroda (Fujitsu) <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: Alexander Lakhin <[email protected]>
Cc: pgsql-hackers <[email protected]>
Cc: Masahiko Sawada <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Slot's restart_lsn may point to removed WAL segment after hard restart unexpectedly
Date: Thu, 19 Jun 2025 15:59:25 +0530
Message-ID: <CAA4eK1+qL9QFOD2Q5kcq0Ff=7OcBLE34QuDBdPcdznUzQwv+eg@mail.gmail.com> (raw)
In-Reply-To: <CAPpHfdv3UEUBjsLhB_CwJT0xX9LmN6U+__myYopq4KcgvCSbTg@mail.gmail.com>
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On Wed, Jun 18, 2025 at 10:17 PM Alexander Korotkov
<[email protected]> wrote:
>
> On Wed, Jun 18, 2025 at 6:50 PM Vitaly Davydov <[email protected]> wrote:
> > > I think, it is a good idea. Once we do not use the generated data, it is ok
> > > just to generate WAL segments using the proposed function. I've tested this
> > > function. The tests worked as expected with and without the fix. The attached
> > > patch does the change.
> >
> > Sorry, forgot to attach the patch. It is created on the current master branch.
> > It may conflict with your corrections. I hope, it could be useful.
>
> Thank you. I've integrated this into a patch to improve these tests.
>
> Regarding assertion failure, I've found that assert in
> PhysicalConfirmReceivedLocation() conflicts with restart_lsn
> previously set by ReplicationSlotReserveWal(). As I can see,
> ReplicationSlotReserveWal() just picks fresh XLogCtl->RedoRecPtr lsn.
> So, it doesn't seems there is a guarantee that restart_lsn never goes
> backward. The commit in ReplicationSlotReserveWal() even states there
> is a "chance that we have to retry".
>
I don't see how this theory can lead to a restart_lsn of a slot going
backwards. The retry mentioned there is just a retry to reserve the
slot's position again if the required WAL is already removed. Such a
retry can only get the position later than the previous restart_lsn.
> Thus, I propose to remove the
> assertion introduced by ca307d5cec90.
>
If what I said above is correct, then the following part of the commit
message will be incorrect:
"As stated in the ReplicationSlotReserveWal() comment, this is not
always true. Additionally, this issue has been spotted by some
buildfarm
members."
--
With Regards,
Amit Kapila.
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Subject: Re: Slot's restart_lsn may point to removed WAL segment after hard restart unexpectedly
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