Re: Report checkpoint progress with pg_stat_progress_checkpoint (was: Report checkpoint progress in server logs) - Mailing list pgsql-hackers

Hi,

On Fri, Feb 18, 2022 at 08:07:05PM +0530, Nitin Jadhav wrote:
> 
> The backend_pid contains a valid value only during
> the CHECKPOINT command issued by the backend explicitly, otherwise the
> value will be 0.  We may have to add an additional field to
> 'CheckpointerShmemStruct' to hold the backend pid. The backend
> requesting the checkpoint will update its pid to this structure.
> Kindly let me know if you still feel the backend_pid field is not
> necessary.

There are more scenarios where you can have a baackend requesting a checkpoint
and waiting for its completion, and there may be more than one backend
concerned, so I don't think that storing only one / the first backend pid is
ok.

> > And also while looking at the patch I see there's the same problem that I
> > mentioned in the previous thread, which is that the effective flags can be
> > updated once the checkpoint started, and as-is the view won't reflect that.  It
> > also means that you can't simply display one of wal, time or force but a
> > possible combination of the flags (including the one not handled in v1).
> 
> If I understand the above comment properly, it has 2 points. First is
> to display the combination of flags rather than just displaying wal,
> time or force - The idea behind this is to just let the user know the
> reason for checkpointing. That is, the checkpoint is started because
> max_wal_size is reached or checkpoint_timeout expired or explicitly
> issued CHECKPOINT command. The other flags like CHECKPOINT_IMMEDIATE,
> CHECKPOINT_WAIT or CHECKPOINT_FLUSH_ALL indicate how the checkpoint
> has to be performed. Hence I have not included those in the view.  If
> it is really required, I would like to modify the code to include
> other flags and display the combination.

I think all the information should be exposed.  Only knowing why the current
checkpoint has been triggered without any further information seems a bit
useless.  Think for instance for cases like [1].

> Second point is to reflect
> the updated flags in the view. AFAIK, there is a possibility that the
> flags get updated during the on-going checkpoint but the reason for
> checkpoint (wal, time or force) will remain same for the current
> checkpoint. There might be a change in how checkpoint has to be
> performed if CHECKPOINT_IMMEDIATE flag is set. If we go with
> displaying the combination of flags in the view, then probably we may
> have to reflect this in the view.

You can only "upgrade" a checkpoint, but not "downgrade" it.  So if for
instance you find both CHECKPOINT_CAUSE_TIME and CHECKPOINT_FORCE (which is
possible) you can easily know which one was the one that triggered the
checkpoint and which one was added later.

> > > Probably a new field named 'processes_wiating' or 'events_waiting' can be
> > > added for this purpose.
> >
> > Maybe num_process_waiting?
> 
> I feel 'processes_wiating' aligns more with the naming conventions of
> the fields of the existing progres views.

There's at least pg_stat_progress_vacuum.num_dead_tuples.  Anyway I don't have
a strong opinion on it, just make sure to correct the typo.

> > > Probably writing of buffers or syncing files may complete before
> > > pg_is_in_recovery() returns false. But there are some cleanup
> > > operations happen as part of the checkpoint. During this scenario, we
> > > may get false value for pg_is_in_recovery(). Please refer following
> > > piece of code which is present in CreateRestartpoint().
> > >
> > > if (!RecoveryInProgress())
> > >         replayTLI = XLogCtl->InsertTimeLineID;
> >
> > Then maybe we could store the timeline rather then then kind of checkpoint?
> > You should still be able to compute the information while giving a bit more
> > information for the same memory usage.
> 
> Can you please describe more about how checkpoint/restartpoint can be
> confirmed using the timeline id.

If pg_is_in_recovery() is true, then it's a restartpoint, otherwise it's a
restartpoint if the checkpoint's timeline is different from the current
timeline?

[1] https://www.postgresql.org/message-id/1486805889.24568.96.camel%40credativ.de