Re: Is there a way (except from server logs) to know the kind of on-going/last checkpoint?

At Thu, 27 Jan 2022 19:09:29 +0800, Julien Rouhaud <rjuju123(at)gmail(dot)com> wrote in
> On Thu, Jan 27, 2022 at 06:56:57PM +0800, Julien Rouhaud wrote:
> >
> > What it's showing is the "currently ongoing checkpoint or last completed
> > checkpoint" kind.
>
> Ah after double checking I see it's storing the information *after* the
> checkpoint completion, so it's indeed the last completed checkpoint. I'm not
> sure how useful it can be, but ok.

I don't see it useful (but don't oppose) to record checkpoint kind in
control file. It is a kind of realtime noncritical info and in the
first place retrievable from server log if needed. And I'm skeptical
that it is needed such frequently. Checkpoint kind is useful to check
max_wal_size's appropriateness if it is in a summarized form as in
pg_stat_bgwriter. On the other hand showing the same in a stats view
or the output of pg_control_checkpoint() is fine by me.

> > Also, it's only showing the initial triggering conditions of checkpoints.
> > For instance, if a timed checkpoint is started and then a backend executes a
> > "CHECKPOINT;", it will upgrade the ongoing checkpoint with additional flags but
> > AFAICS those new flags won't be saved to the control file.
>
> This one is still valid I think, it's only storing the initial flags and not
> the possibly upgraded one in shmem.

Agreed.

I don't like to add this complex-but-need-in-sync blob twice. If we
need to do that twice, I want them consolidated in any shape.

> Datum values[18];
> bool nulls[18];

You forgot to expand these arrays.

This breaks checkpoint file format. Need to bump PG_CONTROL_VERSION,
and pg_upgrade need to treat the change.

Even if we add checkpoint kind to control file, it would look a bit
strange that the "checkpoint kind" shows first among all
checkpoint-related lines. And at least the "wait" in the line is
really useless since it is not a property of a checkpoint. Instead, it
doesn't show "requested" which is one of the checkpoint properties
like "xlog" and "time". I'm not sure we need all of the properties to
be shown but I don't have a clear criteria for each property of it
ought to be shown or not.

> pg_control last modified: Fri 28 Jan 2022 09:49:46 AM JST
> Latest checkpoint kind: immediate force wait
> Latest checkpoint location: 0/172B2C8

I'd like to see the PID of the triggering process, but it is really
not a information suitable in the control file...

- proallargtypes => '{pg_lsn,pg_lsn,text,int4,int4,bool,text,oid,xid,xid,xid,oid,xid,xid,oid,xid,xid,timestamptz}',
+ proallargtypes => '{pg_lsn,pg_lsn,text,int4,int4,bool,text,oid,xid,xid,xid,oid,xid,xid,oid,xid,xid,timestamptz,text}',

I think the additional column should be text[] instead of text, but
not sure.

And you need to edit the corresponding part of the doc.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center