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

On Wed, Feb 9, 2022 at 7:49 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> Honestly, I never understood why the checkpoint during CREATE DATABASE
> was as problem --- we checkpoint by default every five minutes anyway,
> so why is an additional two a problem --- it just means the next
> checkpoint will do less work. It is hard to see how avoiding
> checkpoints to add WAL writes, fscyncs, and replication traffic could be
> a win.

But don't you think that the current way of WAL logging the CREATE
DATABASE is a bit hacky? I mean we are just logically WAL logging the
source and destination directory paths without actually WAL logging
what content we want to copy. IMHO this is against the basic
principle of WAL and that's the reason we are forcefully checkpointing
to avoid replaying that WAL during crash recovery. Even after this
some of the code comments say that we have limitations during PITR[1]
and we want to avoid it sometime in the future.

[1]
* In PITR replay, the first of these isn't an issue, and the second
* is only a risk if the CREATE DATABASE and subsequent template
* database change both occur while a base backup is being taken.
* There doesn't seem to be much we can do about that except document
* it as a limitation.
*
* Perhaps if we ever implement CREATE DATABASE in a less cheesy way,
* we can avoid this.
*/
RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com