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

On 12/7/21 15:36, Bharath Rupireddy wrote:
> Hi,
>
> Currently one can know the kind of on-going/last checkpoint (shutdown,
> end-of-recovery, immediate, force etc.) only via server logs that to
> when log_checkpoints GUC is on. At times, the users/service layer
> components would want to know the kind of checkpoint (along with other
> checkpoint related info) to take some actions and it will be a bit
> difficult to search through the server logs. The checkpoint info can
> be obtained from the control file (either by pg_control_checkpoint()
> or by pg_controldata tool) whereas checkpoint kind isn't available
> there.
>
> How about we add an extra string field to the control file alongside
> the other checkpoint info it already has? This way, the existing
> pg_control_checkpoint() or pg_controldata tool can easily be enhanced
> to show the checkpoint kind as well. One concern is that we don't want
> to increase the size of pg_controldata by more than the typical block
> size (of 8K) to avoid any torn-writes. With this change, we might add
> at max the strings specified at [1]. Adding it to the control file has
> an advantage of preserving the last checkpoint kind which might be
> useful.
>
> Thoughts?
>

I agree it might be useful to provide information about the nature of
the checkpoint, and perhaps even PID of the backend that triggered it
(although that may be tricky, if the backend terminates).

I'm not sure about adding it to control data, though. That doesn't seem
like a very good match for something that's mostly for monitoring.

We already have some checkpoint info in pg_stat_bgwriter, but that's
just aggregated data, not very convenient for info about the current
checkpoint. So maybe we should add pg_stat_progress_checkpoint, showing
various info about the current checkpoint?

> [1] for checkpoint: "checkpoint shutdown end-of-recovery immediate
> force wait wal time flush-all"
> for restartpoint: "restartpoint shutdown end-of-recovery immediate
> force wait wal time flush-all"
>

I'd bet squashing all of this into a single string (not really a flag)
will just mean people will have to parse it, etc. Keeping individual
boolean flags (or even separate string fields) would be better, I guess.

regards

--
Tomas Vondra
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