Re: Possible causes of high_replay lag, given replication settings?

From: Greg Sabino Mullane <htamfids(at)gmail(dot)com>
To: Jon Zeppieri <zeppieri(at)gmail(dot)com>
Cc: Nick Cleaton <nick(at)cleaton(dot)net>, pgsql-general(at)lists(dot)postgresql(dot)org
Subject: Re: Possible causes of high_replay lag, given replication settings?
Date: 2025-07-25 23:12:26
Message-ID: CAKAnmm+jtytmNAYDjBkiQvCHQVwtskUzvp3J_=5mUsFvaOkRVA@mail.gmail.com
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On Fri, Jul 25, 2025 at 9:57 AM Jon Zeppieri <zeppieri(at)gmail(dot)com> wrote:

> Thanks for the response, Nick. I'm curious why the situation you describe
> wouldn't also lead to the write_lag and flush_lag also being
> high. If the problem is simply keeping up with the primary, wouldn't you
> expect all three lag times to be elevated?
>

No - write and flush are pretty quick and simple, it's just putting the WAL
onto the local disk. Replay involves a lot more work as we have to parse
the WAL and apply the changes, which means doing a lot of I/O across many
files. Still, *hours* to me indicates more than just a lot of extra
traffic. Check that recovery_min_apply_delay is still 0, then log onto the
replica and see what's going on with regards to open transactions and locks.

Cheers,
Greg

--
Crunchy Data - https://www.crunchydata.com
Enterprise Postgres Software Products & Tech Support

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