Feedback-ID: id4a34324:Fastmail
Date: Tue, 12 Nov 2024 08:55:37 -0500
From: Andres Freund <andres@anarazel.de>
To: Jan Wieck <jan@wi3ck.info>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Tomas Vondra <tomas@vondra.me>, 
	"Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, Aleksander Alekseev <aleksander@timescale.com>, 
	pgsql-hackers <pgsql-hackers@postgresql.org>, shveta malik <shveta.malik@gmail.com>
Subject: Re: Commit Timestamp and LSN Inversion issue
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Hi,

On 2024-11-12 08:51:49 -0500, Jan Wieck wrote:
> On 11/11/24 23:21, Amit Kapila wrote:
> > As the inversion issue can mainly hamper logical replication-based
> > solutions we can do any of this additional work under spinlock only
> > when the current record is a commit record (which the currently
> > proposed patch is already doing) and "wal_level = logical" and also
> > can have another option at the subscription level to enable this new
> > code path. I am not sure what is best but just sharing the ideas here.
>
> It can indeed be reduced to one extra *unlikely* if test only for commit
> records and only when WAL level is "logical". Without those two being true
> there would be zero impact on ReserveXLogInsertLocation().

No, the impact would be far larger, because it would make it impractical to
remove this bottleneck by using atomic instructions in
ReserveXLogInsertLocation(). It doesn't make sense to make it harder to
resolve one of the core central bottlenecks in postgres for a niche feature
like this. Sorry, but this is just not a viable path.

Greetings,

Andres Freund