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help / color / mirror / Atom feedFrom: Subramanian,Ramachandran <[email protected]>
To: Laurenz Albe <[email protected]>
To: [email protected] <[email protected]>
Subject: AW: Replication Testing- How to introduce a Lag
Date: Tue, 24 Mar 2026 07:51:57 +0000
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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 <[email protected]>
Gesendet: Dienstag, 24. März 2026 08:07
An: Subramanian,Ramachandran IT-md-db <[email protected]>; [email protected]
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
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