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

On Mon, Jun 20, 2016 at 10:08 PM, Bruce Momjian <bruce@momjian.us> wrote:
> On Fri, May 20, 2016 at 07:40:53PM -0400, Robert Haas wrote:
>> On Mon, May 16, 2016 at 3:36 AM, Bruce Momjian <bruce@momjian.us> wrote:
>> > On Sun, May 15, 2016 at 03:23:52PM -0500, Jim Nasby wrote:
>> >> 2) There's no ability at all to revert, other than restore a backup. That
>> >> means if you pull the trigger and discover some major performance problem,
>> >> you have no choice but to deal with it (you can't switch back to the old
>> >> version without losing data).
>> >
>> > In --link mode only
>>
>> 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.