Re: Lets (not) break all the things. Was: [pgsql-advocacy] 9.6 -> 10.0

On Tue, Jun 21, 2016 at 08:19:55AM -0400, Robert Haas wrote:
> On Mon, Jun 20, 2016 at 10:08 PM, Bruce Momjian <bruce@momjian.us> wrote:
> >> No, not really.  Once you let write transactions into the new cluster,
> >> there's no way to get back to the old server version no matter which
> >> option you used.
> >
> > Yes, there is, and it is documented:
> >
> >         If you ran <command>pg_upgrade</command> <emphasis>without</>
> >         <option>--link</> or did not start the new server, the
> >         old cluster was not modified except that, if linking
> >         started, a <literal>.old</> suffix was appended to
> >         <filename>$PGDATA/global/pg_control</>.  To reuse the old
> >         cluster, possibly remove the <filename>.old</> suffix from
> >         <filename>$PGDATA/global/pg_control</>; you can then restart the
> >         old cluster.
> >
> > What is confusing you?
> 
> I don't think I'm confused.  Sure, you can do that, but the effects of
> any writes performed on the new cluster will not be there when you
> revert back to the old cluster.  So you will have effectively lost
> data, unless you somehow have the ability to re-apply all of those
> write transactions somehow.

Yes, that is true.  I assume _revert_ means something really bad
happened and you don't want those writes because they are somehow
corrupt.

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

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