Streaming replication and WAL archive interactions

From: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>
To: PostgreSQL-development <pgsql-hackers(at)postgreSQL(dot)org>
Subject: Streaming replication and WAL archive interactions
Date: 2014-12-12 13:46:51
Message-ID: 548AF1CB.80702@vmware.com
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There have been a few threads on the behavior of WAL archiving, after a
standby server is promoted [1] [2]. In short, it doesn't work as you
might expect. The standby will start archiving after it's promoted, but
it will not archive files that were replicated from the old master via
streaming replication. If those files were not already archived in the
master before the promotion, they are not archived at all. That's not
good if you wanted to restore from a base backup + the WAL archive later.

The basic setup is a master server, a standby, a WAL archive that's
shared by both, and streaming replication between the master and
standby. This should be a very common setup in the field, so how are
people doing it in practice? Just live with the wisk that you might miss
some files in the archive if you promote? Don't even realize there's a
problem? Something else?

And how would we like it to work?

There was some discussion in August on enabling WAL archiving in the
standby, always [3]. That's a related idea, but it assumes that you have
a separate archive in the master and the standby. The problem at
promotion happens when you have a shared archive between the master and
standby.

[1]
http://www.postgresql.org/message-id/CAHGQGwHVYqbX=A+zo+AvFbVHLGoypO9G_QDKbabeXgXBVGd05g@mail.gmail.com

[2] http://www.postgresql.org/message-id/20140904175036.310c6466@erg

[3]
http://www.postgresql.org/message-id/CAHGQGwHNMs-syU=MEVSESTHna+Exd9pfO_OHHFPJCwOVaYRZKw@mail.gmail.com.

- Heikki

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