Subject: Re: Guidance Needed for PostgreSQL Upgrade from 12 to 15
To: pgsql-admin@lists.postgresql.org
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From: Kris Deugau <kdeugau@vianet.ca>
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Date: Tue, 19 Nov 2024 12:32:29 -0500
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Scott Ribe wrote:
>> On Nov 18, 2024, at 12:10 PM, Achilleas Mantzios <a.mantzios@cloud.gatewaynet.com> wrote:
>>
>>
>> I'd like to add if someone can tolerate some seconds of downtime, with logical replication it is possible to swap the roles of publisher / subscriber so that one can keep the old database as a means of extra safety. If his/her app breaks beyond repair and there is no time for fixing, one can simply go back to the old DB with some minor adjustments (such as sequences).
> 
> In the same vein, if one's file system offers atomic snapshots:
> 
> 1) shut down PG
> 2) run pg_upgrade with the hard links option
> 3) take snapshot
> 4) start up new PG
> 
> Now, if there's a failure, you can go back to the snapshot and either start up the old PG, or do some troubleshooting and try upgrading again.

Er...  I would think that you'd want to do:

1) shut down PG
2) take snapshot
3) run pg_upgrade with the hard links option
4) start up new PG

as otherwise the old PG won't start properly, due to the changes made by 
pg_upgrade.

-kgd