Re: Asynchronous and "direct" IO support for PostgreSQL.

Hi,

On 2021-02-24 21:15:14 +0500, Alexey Lesovsky wrote:
> pg_stat_aios.
> In general, this stat is a set of text values, and at the same time it looks
> GAUGE-like (similar to pg_stat_activity or pg_locks), and is only relevant
> for the moment when the user is looking at it. I think it would be better to
> rename this view to pg_stat_progress_aios. And keep pg_stat_aios for other
> AIO stats with global COUNTERs (like stuff in pg_stat_user_tables or
> pg_stat_statements, or system-wide /proc/stat, /proc/diskstats).

Right - arguably it really shouldn't even have _stat_ in the name... I
don't particularly like the idea of adding _progress_ as that seems it'd
lead to confusing it with pg_stat_progress_vacuum etc - and it's quite a
different beast.

> pg_stat_aio_backends.
> This stat is based on COUNTERs, which is great, but the issue here is that
> its lifespan is limited by the lifespan of the backend processes - once the
> backend exits the stat will no longer be available - which could be
> inappropriate in workloads with short-lived backends.

There's a todo somewhere to roll over the per-connection stats into a
global stats piece on disconnect. In addition I was thinking of adding a
view that sums up the value of "already disconnected backends" and the
currently connected ones. Would that mostly address your concerns?

> I think there might be few existing examples in the current code that could
> be repurposed to implement the suggestions above (such as
> pg_stat_user_tables, pg_stat_database, etc). With this in mind, I think
> having these changes incorporated shouldn’t take significant effort
> considering the benefit it will bring to the final user.

Yea - I kind of was planning to go somewhere roughly in the direction
you suggest, but took a few shortcuts due to the size of the
project. Having the views made it a lot easier to debug / develop, but
supporting longer lived stuff wasn't yet crucial. But I agree, we really
should have it...

Greetings,

Andres Freund