On 28 July 2016 at 04:35, Bruce Momjian <bruce@momjian.us> wrote:
On Fri, Jul 8, 2016 at 12:18:28AM +0100, Simon Riggs wrote: > On 7 July 2016 at 21:10, Robert Haas <robertmhaas@gmail.com> wrote: > > pg_upgrade does that, kinda. I'd like to have something better, but > in the absence of that, I think it's quite wrong to think about > deprecating it, even if we had logical replication fully integrated > into core today. Which we by no means do. > > I don't see any problem with extending pg_upgrade to use logical replication > features under the covers. > > It seems very smooth to be able to just say > > pg_upgrade --online > > and then specify whatever other parameters that requires. > > It would be much easier to separate out that as a use-case so we can be sure we > get that in 10.0, even if nothing else lands.
Uh, while "pg_upgrade --online" looks cool, I am not sure a solution based on logical replication would share _any_ code with the existing pg_upgrade tool, so it seems best to use another binary for this.
It might, actually. One approach for online upgrade is to:
* pg_basebackup the master
* start the replica and let it catch up
* create a logical replication slot on the master
* replace the replication.conf on the basebackup so it stops recovery at the lsn of the replication slot's confirmed_flush_lsn
* stop the replica and pg_upgrade it
* have the upgraded replica, now a master, replay from the old master over logical replication
* once caught up, switch over
This means a full dump and reload with a full rebuild of all indexes, etc, isn't needed. All shared catalog stuff is copied (until we switch to logical rep for the final catch-up).
I guess we could use the pg_dump/pg_restore pg_upgrade code to create the objects, and use logical replication to copy the rows, but what does this gain us that pg_dump/pg_restore doesn't?
A consistent switch-over point, where the upgrade can happen while the master is still writing.
We create a slot, dump from the slot's exported snapshot, and switch over to logical replication consistently at the end of the dump.