On Tue, Jun 21, 2016 at 11:34 AM, Bruce Momjian <br...@momjian.us> wrote: > On Tue, Jun 21, 2016 at 08:19:55AM -0400, Robert Haas wrote: >> On Mon, Jun 20, 2016 at 10:08 PM, Bruce Momjian <br...@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.
I think that it's pretty likely you could, say, upgrade to a new major release, discover that it has a performance problem or some other bug that causes a problem for you, and want to go back to the older release. There's not really an easy way to do that, because a pg_dump taken from the new system might not restore on the older one. Logical replication - e.g. Slony - can provide a way, but we don't have anything in core that can do it. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
