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From: Amit Kapila <amit.kapila16@gmail.com>
Date: Tue, 27 Feb 2024 18:30:28 +0530
Message-ID: <CAA4eK1J6wOMkDDt3iOs+JqkvQvuwSORosTpqdejgvXqFmxex=A@mail.gmail.com>
Subject: Re: Slow catchup of 2PC (twophase) transactions on replica in LR
To: =?UTF-8?B?0JTQsNCy0YvQtNC+0LIg0JLQuNGC0LDQu9C40Lk=?= <v.davydov@postgrespro.ru>
Cc: pgsql-hackers@lists.postgresql.org
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On Tue, Feb 27, 2024 at 4:49=E2=80=AFPM =D0=94=D0=B0=D0=B2=D1=8B=D0=B4=D0=
=BE=D0=B2 =D0=92=D0=B8=D1=82=D0=B0=D0=BB=D0=B8=D0=B9
<v.davydov@postgrespro.ru> wrote:
>
> Thank you for your interest in the discussion!
>
> On Monday, February 26, 2024 16:24 MSK, Amit Kapila <amit.kapila16@gmail.=
com> wrote:
>
>
> I think the reason is probably that when the WAL record for prepared is a=
lready flushed then what will be the idea of async commit here?
>
> I think, the idea of async commit should be applied for both transactions=
: PREPARE and COMMIT PREPARED, which are actually two separate local transa=
ctions. For both these transactions we may call XLogSetAsyncXactLSN on comm=
it instead of XLogFlush when async commit is enabled. When I use async comm=
it, I mean to apply async commit to local transactions, not to a twophase (=
prepared) transaction itself.
>
>
> At commit prepared, it seems we read prepare's WAL record, right? If so, =
it is not clear to me do you see a problem with a flush of commit_prepared =
or reading WAL for prepared or both of these.
>
> The problem with reading WAL is due to async commit of PREPARE TRANSACTIO=
N which saves 2PC in the WAL. At the moment of COMMIT PREPARED the WAL with=
 PREPARE TRANSACTION 2PC state may not be XLogFlush-ed yet.
>

As we do XLogFlush() at the time of prepare then why it is not
available? OR are you talking about this state after your idea/patch
where you are trying to make both Prepare and Commit_prepared records
async?

 So, PREPARE TRANSACTION should wait until its 2PC state is flushed.
>
> I did some experiments with saving 2PC state in the local memory of logic=
al replication worker and, I think, it worked and demonstrated much better =
performance. Logical replication worker utilized up to 100% CPU. I'm just c=
oncerned about possible problems with async commit for twophase transaction=
s.
>
> To be more specific, I've attached a patch to support async commit for tw=
ophase. It is not the final patch but it is presented only for discussion p=
urposes. There were some attempts to save 2PC in memory in past but it was =
rejected.
>

It would be good if you could link those threads.

--=20
With Regards,
Amit Kapila.