On Mon, Mar 14, 2016 at 6:28 PM, John Lumby <johnlu...@hotmail.com> wrote:
> > From: johnlu...@hotmail.com > To: pgsql-general-ow...@postgresql.org > Subject: how to switch old replication Master to new Standby after > promoting old Standby > Date: Mon, 14 Mar 2016 13:23:29 -0400 > > In a scenario involving replication where no failure occurs but I want to > interchange Master <->Standby, > *and* want to avoid making another full base backup and rsyncing it across, > I have found it easy to promote old Standby to new Master with pg_ctl > promote, > but very difficult to restart the old Primary as a new Standby *without* > performing new base backup. > > Assume current 9.5.1 and using streaming replication with a named > replication slot if relevant > > Second assumption - I am able to temporarily prevent any relational > updates to the database before I start the switchover > e.g. the > default_transaction_read_only = on > setting together with being able to control what transactions do if > anything > > So I reach a point where both systems have postgresql running without any > replication, > both have identical content in all databases (that I can control), > and I am willing to tolerate short restarts if need be and also to > scp/rsync the contents of pg_xlog > and other small files but *not* the entire cluster directory or any > database base directories. > > What do I do next? > > Here is what I have found seems to work but I am not sure it is robust: > > 1. shut down both new Master and intended-to-be-new-Standby > 2. on intended-to-be-new-Standby, remove the entire content of pg_xlog and > the global/pg_control > 3. from new Master , tar + scp the entire content of pg_xlog and the > global/pg_control to intended-to-be-new-Standby > That does seem like a very risky strategy to me. Have you taken a look at pg_rewind (which is now part of the distribution)? -- Alex
