Re: pg_stat_io not tracking smgrwriteback() is confusing

On 4/19/23 1:23 PM, Andres Freund wrote:
> Hi,
> 
> I noticed that the numbers in pg_stat_io dont't quite add up to what I
> expected in write heavy workloads. Particularly for checkpointer, the numbers
> for "write" in log_checkpoints output are larger than what is visible in
> pg_stat_io.
> 
> That partially is because log_checkpoints' "write" covers way too many things,
> but there's an issue with pg_stat_io as well:
> 
> Checkpoints, and some other sources of writes, will often end up doing a lot
> of smgrwriteback() calls - which pg_stat_io doesn't track. Nor do any
> pre-existing forms of IO statistics.
> 
> It seems pretty clear that we should track writeback as well. I wonder if it's
> worth doing so for 16? It'd give a more complete picture that way. The
> counter-argument I see is that we didn't track the time for it in existing
> stats either, and that nobody complained - but I suspect that's mostly because
> nobody knew to look.

[RMT hat]

(sorry for slow reply on this, I've been out for a few days).

It does sound generally helpful to track writeback to ensure anyone 
building around pg_stat_io can see tthe more granular picture. How big 
of an effort is this? Do you think this helps to complete the feature 
for v16?

Thanks,

Jonathan