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help / color / mirror / Atom feedFrom: Subramanian,Ramachandran <ramachandran.subramanian@alte-leipziger.de>
To: Laurenz Albe <laurenz.albe@cybertec.at>
To: pgsql-novice@lists.postgresql.org <pgsql-novice@lists.postgresql.org>
Subject: AW: AW: AW: AW: AW: Replication Testing- How to introduce a Lag
Date: Tue, 24 Mar 2026 06:14:23 +0000
Message-ID: <aa307627991c41f280bb6da27ebc9b1d@alte-leipziger.de> (raw)
In-Reply-To: <89d63402747edb3bf80ff668fdd63cc77334f446.camel@cybertec.at>
References: <aadb9058660d4e10b5cee44b82458802@alte-leipziger.de>
<f3927bc3a9baf4400325b91652869907f0581c97.camel@cybertec.at>
<e1e41761323040228570cf6a7e02518e@alte-leipziger.de>
<8b38fa5141905f9ad6983b8d5ae944e812e3d28a.camel@cybertec.at>
<fa47fe18078043adbdfe007b7270ffd0@alte-leipziger.de>
<fc42f77c80fdb3c0267148b807edfacc49b0b0ca.camel@cybertec.at>
<fc3be1f56ded49dc9be519f92d8b9ab4@alte-leipziger.de>
<0e00fcc0062b59b88f98dba4900ce49d5a9a3fea.camel@cybertec.at>
<477369bf897b44feb2d2368f6fca93fc@alte-leipziger.de>
<89d63402747edb3bf80ff668fdd63cc77334f446.camel@cybertec.at>
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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To: pgsql-novice@postgresql.org
Cc: ramachandran.subramanian@alte-leipziger.de, laurenz.albe@cybertec.at, pgsql-novice@lists.postgresql.org
Subject: Re: AW: AW: AW: AW: AW: Replication Testing- How to introduce a Lag
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