On Wed, Feb 9, 2022 at 1:34 PM Bruce Momjian <br...@momjian.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
