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

On Wed, Feb 9, 2022 at 1:34 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Well, I think the worst case is that the checkpoint happens exactly
> between two checkpoints, so you are checkpointing twice as often, but if
> it happens just before or after a checkpoint, I assume the effect would
> be minimal.

I agree for the most part. I think that if checkpoints happen every 8
minutes normally and the extra checkpoint happens 2 minutes after the
previous checkpoint, the impact may be almost as bad as if it had
happened right in the middle. If it happens 5 seconds after the
previous checkpoint, it should be low impact.

> So, it seems we are weighing having a checkpoint happen in the middle of
> a checkpoint interval vs writing more WAL. If the WAL traffic, without
> CREATE DATABASE, is high, and the template database is small, writing
> more WAL and skipping the checkpoint will be win, but if the WAL traffic
> is small and the template database is big, the extra WAL will be a loss.
> Is this accurate?

I think that's basically correct. I would expect that the worry about
big template database is mostly about template databases that are
REALLY big. I think if your template database is 10GB you probably
shouldn't be worried about this feature. 10GB of extra WAL isn't
nothing, but if you've got reasonably capable hardware, it's not
overloaded, and max_wal_size is big enough, it's probably not going to
have a huge impact. Also, most of the impact will probably be on the
CREATE DATABASE command itself, and other things running on the system
at the same time will be impacted to a lesser degree. I think it's
even possible that you will be happier with this feature than without,
because you may like the idea that CREATE DATABASE itself is slow more
than you like the idea of it making everything else on the system
slow. On the other hand, if your template database is 1TB, the extra
WAL is probably going to be a fairly big problem.

Basically I think for most people this should be neutral or a win. For
people with really large template databases, it's a loss. Hence the
discussion about having a way for people who prefer the current
behavior to keep it.

> Agreed. We would want to have a different heap/index key on the standby
> so we can rotate the heap/index key.

I don't like that design, and I don't think that's what we should do,
but I understand that you feel differently. IMHO, this thread is not
the place to hash that out.

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