What are you looking at to claim the number of connections is under 600? Maybe there's some disconnect between what you're measuring and what the database thinks.
A different line of thought is that ProcArray slots can be consumed by things that aren't client connection processes, in particular (1) parallel-query workers (2) autovacuum workers Looking into pg_stat_activity when you see this issue might help clarify that.
Sorry, I could have been clearer - pg_stat_activity is what I'm looking at - I'm recording connection info from here every 15 seconds (from a superuser account so I don't get locked out). It never peaks above 300 (in fact when the incident happens no new connections can come in so it falls to around 100) - yet I'm seeing the log lines claim that I'm still hitting 597 (600 - 3 reserved).
If I measure from `ps -ef | grep postgres` and look at the connections then I can see that with the startup connections I am hitting this limit.
So client processes which are listed to the OS as "startup" ARE counted towards the 597 connections, but are NOT reported in pg_stat_activity
James
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