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 




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i. A. Ramachandran Subramanian
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-----Ursprüngliche Nachricht-----
Von: Laurenz Albe <laurenz.albe@cybertec.at> 
Gesendet: Dienstag, 24. März 2026 08:07
An: Subramanian,Ramachandran IT-md-db <ramachandran.subramanian@alte-leipziger.de>; pgsql-novice@lists.postgresql.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