Date: Fri, 12 Mar 2021 14:47:08 -0500
From: Bruce Momjian <bruce@momjian.us>
To: tsuraan <tsuraan@gmail.com>
Cc: pgsql-admin@postgresql.org
Subject: Re: Streaming replication upgrade sanity check
Message-ID: <20210312194708.GB5362@momjian.us>
References: 
 <CALKcMwLoZajZ8_8OJefNgaCiaXa0MsbDFiiWN-nYyGD-9gAbiA@mail.gmail.com>
 <20210312005016.GV20766@tamriel.snowman.net>
 <20210312005557.GA2999@momjian.us>
 <CALKcMw+Y0HRFHY8uNS+VGKeFmbnCVJ02Az1YK4r6-_FnXgPDWg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: 
 <CALKcMw+Y0HRFHY8uNS+VGKeFmbnCVJ02Az1YK4r6-_FnXgPDWg@mail.gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Archived-At: 
 <https://www.postgresql.org/message-id/20210312194708.GB5362%40momjian.us>
Precedence: bulk

On Fri, Mar 12, 2021 at 10:53:11AM -0600, tsuraan wrote:
> > He/she wants to run the standby, I guess in read-only mode, while
> > pg_upgrade is running, which is why I didn't even bother to mention
> > that.  I think if downtime is the critical for this person, he/she
> > should be using logical replication for the upgrade.  pg_upgrade really
> > wants full control of the primary/standbys during its operation, and if
> > you can't do that, pg_upgrade is not the right tool to use.
> 
> This thread blew up a bit overnight, so I guess I might be getting a
> bit repetitive, but it's less about super low downtime and more about
> just not having all that much control over everything. The systems are
> installed all over the world, and the upgrade process is very solid
> and well understood (by my client's devs), but it's pretty reliant on
> the main database being running, and also standby systems are fairly
> independent, so doing a lot of coordination is somewhere between
> complicated and maybe not entirely possible.
> 
> I do just want to clarify the bit about pg_upgrade wanting full
> control over standbys. I'm looking over the page again, and it does
> list some prereqs to make sure things will run smoothly on the
> standby, but if I do just abandon the standby systems and do a new
> pg_basebackup on them once the master is running again, that's not
> going to cause any issues on the master, right? I haven't had any
> explosions in testing, but none of my test systems are anywhere near
> as large or busy as the larger customers, so I definitely could be
> missing things.

Right, just making new standbys is simple.  The basic idea is that
pg_upgrade will complain about anything that isn't exactly as required,
to avoid invisible upgrade failures.  When you started asking about
exactly how we could stretch pg_upgrade, we started to have to discuss
exactly what we could relax with 100% reliability, and we didn't come up
with many options.  We are happy to continue that discussion but we
don't have any good ideas right now.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee