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
introduceddelay 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
replayis 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
Zentralbereich Informationstechnologie

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