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help / color / mirror / Atom feedFrom: Laurenz Albe <laurenz.albe@cybertec.at>
To: Subramanian,Ramachandran <ramachandran.subramanian@alte-leipziger.de>
To: pgsql-novice@lists.postgresql.org <pgsql-novice@lists.postgresql.org>
Subject: Re: Replication Testing- How to introduce a Lag
Date: Tue, 24 Mar 2026 08:07:00 +0100
Message-ID: <013104103e016e84d59c1e2accb0b1b1f99d9870.camel@cybertec.at> (raw)
In-Reply-To: <aa307627991c41f280bb6da27ebc9b1d@alte-leipziger.de>
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>
<aa307627991c41f280bb6da27ebc9b1d@alte-leipziger.de>
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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To: pgsql-novice@postgresql.org
Cc: laurenz.albe@cybertec.at, ramachandran.subramanian@alte-leipziger.de, pgsql-novice@lists.postgresql.org
Subject: Re: Replication Testing- How to introduce a Lag
In-Reply-To: <013104103e016e84d59c1e2accb0b1b1f99d9870.camel@cybertec.at>
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