AW: AW: AW: AW: AW: Replication Testing- How to introduce a Lag

By force of habit I used a mainframe term. I am sorry . RBA = Relative Byte Address. Used as Log Sequence Number.

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 ) .

psql -h $SOURCE_HOST -p $SOURCE_PORT -c "select pg_wal_lsn_diff(pg_current_wal_lsn(),replay_lsn) from pg_stat_replication"

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

LG

Ram
[Subramanian,Ramachandran IT-md-db]

On Mon, 2026-03-23 at 16:36 +0000, Subramanian,Ramachandran wrote:
> I noticed that  RBAs are not incremented one for one . i.e  1 row
> inserted does not mean RBA=RBA+1 . 1 row updated  does not mean
> RBA=RBA+1
>
> I have ALTER SYSTEM SET recovery_min_apply_delay=300000 ; ( on the
> stand by side )
>
> On the Source side
> A simple create table results in a RBA difference of 108328
>
> A simple insert of 1 row results in a RBA difference of 296  
> sometimes 96
>
> Is there a way to estimate roughly the amount of data that remains to
> be transfered ?

I don't know what an RBA is...

If you are using recovery_min_apply_delay, don't measure the replication lag with regard to the replay_lsn, because replay is deliberately delayed.
Instead, measure the difference to flush_lsn, the WAL position successfully transferred to the standby and persisted there.

Yours,
Laurenz Albe

Freundliche Grüße

i. A. Ramachandran Subramanian
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