Re: Report checkpoint progress with pg_stat_progress_checkpoint (was: Report checkpoint progress in server logs)

On Sat, Feb 26, 2022 at 02:30:36AM +0800, Julien Rouhaud wrote:
> On Fri, Feb 25, 2022 at 06:49:42PM +0100, Matthias van de Meent wrote:
> >
> > The point I was trying to make was "If cps->ckpt_flags is
> > CHECKPOINT_IMMEDIATE, we hurry up to start the new checkpoint that is
> > actually immediate". That doesn't mean that this checkpoint was
> > created with IMMEDIATE or running using IMMEDIATE, only that optional
> > delays are now being skipped instead.
> 
> Ah, I now see what you mean.
> 
> > To let the user detect _why_ the optional delays are now being
> > skipped, I propose not to report this currently running checkpoint's
> > "flags | CHECKPOINT_IMMEDIATE", but to add reporting of the next
> > checkpoint's flags; which would allow the detection and display of the
> > CHECKPOINT_IMMEDIATE we're actually hurrying for (plus some more
> > interesting information flags.
> 
> I'm still not convinced that's a sensible approach.  The next checkpoint will
> be displayed in the view as CHECKPOINT_IMMEDIATE, so you will then know about
> it.  I'm not sure that having that specific information in the view is
> going to help, especially if users have to understand "a slow checkpoint is
> actually fast even if it's displayed as slow if the next checkpoint is going to
> be fast".  Saying "it's timed" (which imply slow) and "it's fast" is maybe
> still counter intuitive, but at least have a better chance to see there's
> something going on and refer to the doc if you don't get it.

Just to be clear, I do think that it's worthwhile to add some information that
some backends are waiting for that next checkpoint.  As discussed before, an
int for the number of backends looks like enough information to me.