Re: Using old master as new replica after clean switchover

On Thu, 21 Feb 2019 15:38:21 -0300
Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:

> On Tue, Feb 19, 2019 at 9:44 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> >
> > On Tue, Feb 19, 2019 at 04:27:02PM -0800, RSR999GMAILCOM wrote:
> > > So wanted to clarify if this procedure really requires the WAL archive
> > > location on a shared storage ?
> >
> > Shared storage for WAL archives is not a requirement. It is perfectly
> > possible to use streaming replication to get correct WAL changes.
> > Using an archive is recommended for some deployments and depending on
> > your requirements and data retention policy, still you could have
> > those archives on a different host and have the restore_command of the
> > standbyt in recovery or the archive_command of the primary save the
> > segments to it. Depending on the frequency new WAL segments are
> > generated, this depends of course.
>
> If I'm not mistaken, if you don't have WAL archive set up (a shared
> filesystem isn't necessary, but the standby has to be able to restore
> WAL segments from the archive), a few transactions that haven't been
> streamed at primary shutdown could be lost, since the secondary won't
> be able to stream anything after the primary has shut down.

This has been fixed in 9.3. The primary node wait for all WAL records to be
streamed to the connected standbys before shutting down. Including its shutdown
checkpoint. See:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=985bd7d49726c9f178558491d31a570d47340459

Because a standby could disconnect because of some failure during the shutdown
process, you still need to make sure the standby-to-be-promoted received the
shutdown checkpoint though.

> WAL archive can always be restored even without a primary running, hence
> why a WAL archive is needed.

No. Primary does not force a WAL switch/archive during shutdown.

--
Jehan-Guillaume de Rorthais
Dalibo