On 21 June 2016 at 20:19, Robert Haas <robertmh...@gmail.com> wrote: > On Mon, Jun 20, 2016 at 10:08 PM, Bruce Momjian <br...@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 <br...@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. >
Also, if you run *with* --link, IIRC there's no guarantee that the old version will be happy to see any new infomask bits etc introduced by the new Pg. I think there will also be issues with oid to relfilenode mappings in pg_class if the new cluster did any VACUUM FULLs or anything. It seems likely to be a bit risky to fall back on the old cluster once you've upgraded with --link . TBH it never even occurred to me that it'd be possible at all until you mentioned. I always thought of pg_upgrade as a one-way no-going-back process either way, really. Either due to a fork in history (without --link) or due to possibly incompatible datadir changes (with --link). -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
