Re: pg_stat_io clarifications: background worker, writes and reads

Hi

background workers are used to perform tasks on db e.g. I/O, replication,
clone schema, vacuum etc, you can find more details on following links.

https://www.postgresql.org/docs/current/bgworker.html#:~:text=PostgreSQL%20can%20be%20extended%20to,linked%20to%20the%20server's%20status
.

Regards
Kashif Zeeshan
Bitnine

On Wed, May 15, 2024 at 1:26 AM Dimitrios Apostolou <jimis(at)gmx(dot)net> wrote:

> Hello list,
>
> what is the "background worker" in the pg_stat_io statistics view? I'm
> reading the documentation but can't figure this one out knowing that it is
> not autovacuum or bgwriter. And I'm not aware of any extension I might
> have with registered background worker.
>
> Additionally, how can it be evictions > writes? I would expect every
> eviction to cause a write.
>
> Finally about "hits", I understand they are reads found in shared_buffers,
> so they never registered into the "reads" counter. So is "reads" in
> pg_stat_io the equivalent to misses, i.e. the opposite of "hits", the read
> attempts not found in the shared_buffers, that needed to be fetched from
> the disk (or OS buffercache)?
>
> backend_type | object | context | reads | read_time |
> writes | write_time | writebacks | writeback_time | extends | extend_time |
> op_bytes | hits | evictions | reuses | fsyncs | fsync_time |
> stats_reset
>
> -------------------+---------------+---------+---------+-------------+--------+------------+------------+----------------+---------+-------------+----------+-----------+-----------+--------+--------+------------+-------------------------------
> background worker | relation | normal | 5139575 | 2196288.011 |
> 63277 | 1766.94 | 0 | 0 | 0 | 0 |
> 8192 | 876913705 | 5139653 | | 0 | 0 | 2024-04-08
> 08:50:02.971192+00
>
>
> Thank you in advance,
> Dimitris
>
>
>
>