Subject: Re: Performance for initial copy when using pg_logical to upgrade
 Postgres
To: "Westwood, Giles" <g.westwood@orcid.org>,
 pgsql-performance@lists.postgresql.org
References: 
 <CAEMahfP0FCUUuFrDY7gpacwSRp_jM7eNCA__kbVeMupWShE=8A@mail.gmail.com>
From: Andrew Dunstan <andrew@dunslane.net>
Message-ID: <8b064d7e-204f-9a89-24a1-e856c66d408d@dunslane.net>
Date: Fri, 24 Sep 2021 12:00:42 -0400
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On 9/24/21 10:28 AM, Westwood, Giles wrote:
> At Orcid we're trying to upgrade our Postgres database (10 to 13)
> using pg_logical for no downtime. The problem we have is how long the
> initial copy is taking for the ~500GB database. If it takes say 20days
> to complete, will we need to have 20days of WAL files to start
> catching up when it's complete?
>
> I read an earlier thread which pointed me to the tool
> pglogical_create_subscriber which does a pg_basebackup to start the
> initial replication but this is only going to be useful for logical
> clusters on the same version.
>
> I had hoped that the COPY could be parallelized more by
> "max_sync_workers_per_subscription" which is set to 2. However there's
> only a single process:-
>
> postgres 1022196  6.0 24.5 588340 491564 ?       Ds   Sep22 193:19
> postgres: main: xxx xxxx 10.xx.xx.xx(59144) COPY
>
> One of the best resources I've found of real world examples are thead
> on gitlabs own gitlab about their Postgres migrations. They discussed
> one method that might work:-
>
> 1. Setup 9.6 secondary via streaming
> 2. Turn physical secondary into logical secondary
> 3. Shutdown and upgrade secondary to 10
> 4. Turn secondary back on.
>
> In which case we would only need the time required to perform the upgrade.


If you're using the pglogical extension, the best way is often to create
the replica as a physical replica (using pg_basebackup for example), and
then using the extension's utility program pglogical_create_subscriber
to convert the physical replica to a logical replica, which you then
upgrade and switch over to.


Of course, test it out before doing this for real.


cheers


andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com