Re: [HACKERS] Broken hint bits (freeze)

On Mon, Jun 12, 2017 at 06:31:11PM +0300, Vladimir Borodin wrote:
> What about the following sequence?
> 
> 1. Run pg_upgrade on master,
> 2. Start it in single-user mode and stop (to get right wal_level in
> pg_control),
> 3. Copy pg_control somewhere,
> 4. Start master, run analyze and stop.
> 5. Put the control file from step 3 to replicas and rsync them according to the
> documentation.
> 
> And I think that step 10.f in the documentation [1] should be fixed to mention
> starting in single-user mode or with disabled autovacuum.
> 
> [1] https://www.postgresql.org/docs/devel/static/pgupgrade.html

First, I want to apologize for not getting involved in this thread
earlier, and I want to thank everyone for the huge amount of detective
work in finding the cause of this bug.

Let me see if I can replay how the standby server upgrade instructions
evolved over time.

Initially we knew that we had to set wal_level to replica+ so that when
you reconnect to the standby servers, the WAL would have the right
contents.  (We are basically simulating pg_start/stop backup with
rsync.)  

There was a desire to have those instructions inside a documentation
block dedicated to standby server upgrades, so the wal_level adjustment
and new server start/stop was added to that block.  I assumed a
start/stop could not modify the WAL, or at least nothing important would
happen, but obviously I was wrong.  (pg_upgrade takes steps to ensure
that nothing happens.)  Adding ANALYZE in there just made it worse, but
the problem always existed.  I sure hope others haven't had a problem
with this.

Now, it seems we later added a doc section early on that talks about
"Verify standby servers" so I have moved the wal_level section into that
block, which should be safe.  There is now no need to start/stop the new
server since pg_upgrade will do that safely already.

I plan to patch this back to 9.5 where these instructions were added.  I
will mention that this should be in the minor release notes.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

+ As you are, so once was I.  As I am, so you will be. +
+                      Ancient Roman grave inscription +

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