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

I guess what I would be looking for in stats would be a way to tell
what the bandwidth, latency, and queue depth is. Ideally one day
broken down by relation/index and pg_stat_statement record.

I think seeing the actual in flight async requests in a connection is
probably not going to be very useful in production. It's very low
level and in production the user is just going to find that
overwhelming detail. It is kind of cool to see the progress in
sequential operations but I think that should be solved in a higher
level way than this anyways.

What we need to calculate these values would be the kinds of per-op
stats nfsiostat uses from /proc/self/mountstats:
https://utcc.utoronto.ca/~cks/space/blog/linux/NFSMountstatsNFSOps

So number of async reads we've initiated, how many callbacks have been
called, total cumulative elapsed time between i/o issued and i/o
completed, total bytes of i/o initiated, total bytes of i/o completed.
As well a counter of requests which returned errors (eof? i/o error?)
If there are other locks or queues internally to postgres total time
spent in those states.

I have some vague idea that we should have a generic infrastructure
for stats that automatically counts things associated with plan nodes
and automatically bubbles that data up to the per-transaction,
per-backend, per-relation, and pg_stat_statements stats. But that's a
whole other ball of wax :)