From: Arseny Sher <a.sher@postgrespro.ru>
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Cc: pgsql-docs@postgresql.org
Subject: Re: Synchronous logical replication?
References: <877ezznax1.fsf@ars-thinkpad>
	<7fb9fc7c-72f7-f9ef-3896-9cc7e4fd13b0@2ndquadrant.com>
Date: Sun, 02 Jul 2017 11:27:15 +0300
In-Reply-To: <7fb9fc7c-72f7-f9ef-3896-9cc7e4fd13b0@2ndquadrant.com> (Peter
	Eisentraut's message of "Sun, 2 Jul 2017 00:19:44 -0400")
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Peter Eisentraut <peter.eisentraut@2ndquadrant.com> writes:

> The reason for this confusion is that synchronous_commit has both a
> local and a remote meaning, and in this case the local meaning on the
> subscriber has an impact on the remote meaning of the publisher.

And another, if I get it right, is that while asynchronous commit makes
server report 'success' faster, it also makes the actual WAL writing to
the disk *slower* in terms of latency, because this action is delayed
for some arbitrary time (well, to be exactly, up to 3*wal_writer_delay
milliseconds). As for synchronous commit, its WAL is written as soon as
possible, though I am not sure which guarantees exist here -- walwriter
is woken up each wal_writer_delay ms, but it seems to write only fully
completed pages under heavy load. Please correct me if I am wrong.

In general, it's clear now, thanks.

--
Arseny Sher


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