Re: how to switch old replication Master to new Standby after promoting old Standby - Mailing list pgsql-general

Many thanks Alex,
I had not noticed pg_rewind before.   The name pg_rewind is not exactly very descriptive
of what this utility does.    But it is exactly what I need,  thank-you.

I tried it an eventually got it to work but did hit one strange problem -
I ran it like this
pg_rewind -D /mnt/bluebild/pgrepl95 --source-server=host=10.19.0.21 port=5432 user=postgres password=postgres -P
--debug

after which my postgresql server would not start and I discovered that the rewind had actually copied the source's
postgresql.conf,
replacing the one on the target (i.e. the old primary).  

And indeed in its debug I found

received chunk for file "postgresql.conf", offset 0, size 16482
received chunk for file "postgresql.conf.20160314114055", offset 0, size 16464

And I now see in its description in the Doc that it intends to do this.
But why would it do that?


Maybe a note about it should be added to the wiki
https://wiki.postgresql.org/wiki/Streaming_Replication
    (not sure if I can)

Cheers,  and thanks,    John
________________________________
> From: oleksandr.shulgin@zalando.de
> Date: Mon, 14 Mar 2016 18:54:11 +0100
> Subject: Re: [GENERAL] how to switch old replication Master to new
> Standby after promoting old Standby
> To: johnlumby@hotmail.com
> CC: pgsql-general@postgresql.org
>
> On Mon, Mar 14, 2016 at 6:28 PM, John Lumby
> <johnlumby@hotmail.com<mailto:johnlumby@hotmail.com>> wrote:
>
> From: johnlumby@hotmail.com<mailto:johnlumby@hotmail.com>
> To:
> pgsql-general-owner@postgresql.org<mailto:pgsql-general-owner@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
>