AW: Replication Testing- How to introduce a Lag

Thank you for your reply. I am grateful for your knowledge.

My aim is to be able to see

1. Is replication caught up --> No problems here
2. If not caught up , what is the LSN difference --> No problems here , you have already helped me
3. If not caught up, how many bytes / KB worth of data needs to be replicated

So far my replication runs like a dream, I am just preparing for when we transfer our mainframe tables to Postgres and I should be able to answer such questions.

I am just preparing myself.

LG

Ram

Freundliche Grüße

i. A. Ramachandran Subramanian
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-----Ursprüngliche Nachricht-----
Von: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
Gesendet: Dienstag, 24. März 2026 08:07
An: Subramanian,Ramachandran IT-md-db <ramachandran(dot)subramanian(at)alte-leipziger(dot)de>; pgsql-novice(at)lists(dot)postgresql(dot)org
Betreff: Re: Replication Testing- How to introduce a Lag

On Tue, 2026-03-24 at 06:14 +0000, Subramanian,Ramachandran wrote:
> I noticed that if I insert one row in a table at the source, the
> difference in LSNs is not 1 .  ( with a delibrately introduced delay
> on the apply side ),
>
> It is sometimes 96, sometimes 296 ( for the same table two inserts  ) .

Right, because the LSN is not a counter that increases with each new WAL record. It is a position in the WAL stream. The difference between the LSNs of two adjacent WAL records is not 1, but the byte count of the first WAL record.

For example: if you insert a larger row, the LSN will advance more.
Note also that not all inserts will produce the same kind of WAL:
one insert might write a full page image to the WAL, while the next a normal insert record.

> Is there a method to calculate the APPROXIMATE amount of data in (
> Bytes ) that are yet to be transfered from Source to Standby ?

That's exactly what pg_wal_lsn_diff() does.

What is your worry? What is your ultimate goal?

Yours,
Laurenz Albe