Streaming Replication replay lag

From: Nikhil Shetty <nikhil(dot)dba04(at)gmail(dot)com>
To: pgsql-admin(at)postgresql(dot)org
Subject: Streaming Replication replay lag
Date: 2020-11-24 14:24:40
Message-ID: CAFpL5Vzpj7nWvpt-v_2-_DdywQ9vZjh0T5L6gPjcs2fuB7L8NQ@mail.gmail.com
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Hi,

We have one Primary and three Standby Postgresql setup. Two standby in DC
and one standby in DR.

We have configured streaming replication.When there are a lot of write
transactions in the master, we observed that DR lags behind the master for
5-6 hours. We initially thought this could be because of the slow network
between DC and DR and this is normal but when we saw the replay_lsn and
receive_lsn , they had a huge difference.

DR standby was receiving the WAL's in time and there was no delay, the
delay was in replaying the lag. Can anybody point me to where I can start
with the investigation?
DC (Master and 2 Standby) and DR(Standby) have the same setup in terms of
storage and some database configuration parameters but DR has a little less
CPU and RAM.

Attaching replay and receive wal difference.

Attachment Content-Type Size
image/png 69.1 KB

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