Re: log_checkpoint's "WAL file(s) added" is misleading to the point of uselessness

On 2021/07/25 7:50, Andres Freund wrote:
> Hi,
>
> I've been repeatedly confused by the the number of WAL files supposedly
> added. Even when 100s of new WAL files are created the relevant portion
> of log_checkpoints will only ever list zero or one added WAL file.
>
> The reason for that is that CheckpointStats.ckpt_segs_added is only
> incremented in PreallocXlogFiles(). Which has the following comment:
> * XXX this is currently extremely conservative, since it forces only one
> * future log segment to exist, and even that only if we are 75% done with
> * the current one. This is only appropriate for very low-WAL-volume systems.
>
> Whereas in real workloads WAL files are almost exclusively created via
> XLogWrite()->XLogFileInit().
>
> I think we should consider just removing that field. Or, even better, show
> something accurate instead.

+1 to show something accurate instead.

It's also worth showing them in monitoring stats view like pg_stat_wal?

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION