Date: Wed, 19 Aug 2026 14:13:01 +0200
From: hubert depesz lubaczewski <depesz@depesz.com>
Sender: depesz@depesz.com
To: Simon Musila - Localhost <smusila1@gmail.com>
Cc: pgsql-admin@lists.postgresql.org
Subject: Re: PostgreSQL Replication =?utf-8?Q?Lag_?= =?utf-8?B?4oCT?= WALs
 Streaming but Not Being Applied
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On Tue, Aug 18, 2026 at 09:03:39PM +0300, Simon Musila - Localhost wrote:
> *My questions are:*
>    1. What could cause WALs to be successfully streamed to the standby but
>    not applied/replayed?

Concurrent "traffic" (queries). Or special operations in wal that take
"a while" to apply.

>    2. What are the best steps to identify what the WAL replay process is
>    waiting on?

I'd start with:
a. `ps uwf -u postgres` shell command (as long as the underlying os is
   linux-ish.
b. `select * from pg_stat_activity`
c. `sudo strace -f -tt -T -s 256 -p PID-OF-PG-STARTUP-PROCESS` - again,
   on linux hosts
d. pg_waldump of the wal file that takes long time, and check what is
   going on there

Best regards,

depesz