Re: BUG #14109: pg_rewind fails to update target control file in one scenario

Thanks Michael,

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> Date: Tue, 26 Apr 2016 08:04:58 +0900
> Subject: Re: [BUGS] BUG #14109: pg_rewind fails to update target control file in one scenario
> From: michael(dot)paquier(at)gmail(dot)com
> To: johnlumby(at)hotmail(dot)com
> CC: julien(dot)rouhaud(at)dalibo(dot)com; pgsql-bugs(at)postgresql(dot)org
>
> On Tue, Apr 26, 2016 at 7:15 AM, John Lumby <johnlumby(at)hotmail(dot)com> wrote:
>> So, assuming this is reliable (will work without requiring archiving)
>> then my problem is solved.
>
> Depending on the checkpoint frequency and the activity on your
> systems, you may face problems with missing WAL segments at some point
> because past WAL segments need to be recycled or removed by the server
> to move on with its life.

Yes,  I fear I could be caught out by that --
in fact that is why I now always "stabilize" the replication by halting ins/upd/del activity
and then shut the current Primary down first before promoting current Standby.
I *think* that then should guarantee there cannot be any missing WAL segments
when I then rewind the old Primary to become new Standby.

> One way to take care of this class of
> problems is to use wal_keep_segments. An even better one is called
> replication slot.

Regarding replication slots  --   Actually I do use them (I think it is unsafe to run
streaming replication without either archiving or a replication slot)
but even that would still not guarantee success
if I did not take the precaution of shutting down current primary first before flip.

And  ..   we discussed this very point in pqsql-general just a month ago  --

http://www.postgresql.org/message-id/COL131-W804D45E77B0D0FB1EF08B1A3890@phx.gbl

I did not get any answer to my suggestion in that post but I think it might be useful.

> This solely depends on how your system is working,
> so perhaps you will not need some extra configuration.
> --
> Michael

I think there needs to be some clear instructions on exactly what configuration is needed
to be able to run streaming replication and always be able to flip
Standby->Primary   ,  <some actions>   ,   Primary-> Standby

and in those posts in pgsql-general I wrote a suggested addition to the wiki page
but was unable to edit it myself.

Cheers,    John