Re: [Proposal] Fully WAL logged CREATE DATABASE - No Checkpoints

On Thu, Feb 10, 2022 at 2:52 AM Julien Rouhaud <rjuju123(at)gmail(dot)com> wrote:
> Those extra WALs will also impact backups and replication. You could have
> fancy hardware, a read-mostly workload and the need to replicate over a slow
> WAN, and in that case the 10GB could be much more problematic.

True, I guess, but how bad does your WAN have to be for that to be an
issue? On a 1 gigabit/second link, that's a little over 2 minutes of
transfer time. That's not nothing, but it's not extreme, either,
especially because there's no sense in querying an empty database.
You're going to have to put some stuff in that database before you can
do anything meaningful with it, and that's going to have to be
replicated with or without this feature.

I am not saying it couldn't be a problem, and that's why I'm endorsing
making the behavior optional. But I think that it's a niche scenario.
You need a bigger-than-normal template database, a slow WAN link, AND
you need the amount of data loaded into the databases you create from
the template to be small enough to make the cost of logging the
template pages material. If you create a 10GB database from a template
and then load 200GB of data into it, the WAL-logging overhead of
creating the template is only 5%.

I won't really be surprised if we hear that someone has a 10GB
template database and likes to make a ton of copies and only change
500 rows in each one while replicating the whole thing over a slow
WAN. That can definitely happen, and I'm sure whoever is doing that
has reasons for it which they consider good and sufficient. However, I
don't think there are likely to be a ton of people doing stuff like
that - just a few.

--
Robert Haas
EDB: http://www.enterprisedb.com