Re: pg_stat_io clarifications: background worker, writes and reads

Hi

parallel worker are used for parallel execution of the queries and you can
find the help in the below link.

https://www.postgresql.org/docs/current/how-parallel-query-works.html

Its controlled by following parameters.

max_worker_processes = 6
max_parallel_workers_per_gather = 3
max_parallel_workers = 6

The limit of concurrent parallel workers for the whole cluster is
max_parallel_workers, which must be ≤ max_worker_processes. The limit of
parallel workers per query is max_parallel_workers_per_gather.

Thanks
Kashif Zeeshan
Bitnine Global

On Wed, May 15, 2024 at 5:59 PM Dimitrios Apostolou <jimis(at)gmx(dot)net> wrote:

> So what is this particular "background worker" I'm seeing, given that I
> have no replication or extensions?
>
> Searching the logs I found entries like the following:
>
> LOG: background worker "parallel worker" (PID 93384) exited with exit
> code 1
>
> This got logged when I killed a simple SELECT query that took too long
> doing parallel seqscans. Could it be that the entry in pg_stat_io named
> "background worker" also includes the parallel workers from a SELECT
> query?
>
> Thanks,
> Dimitris
>
> On Wed, 15 May 2024, Muhammad Imtiaz wrote:
>
> > Hi,
> >
> > In PostgreSQL, the pg_stat_io view provides detailed statistics on I/O
> operations. Background process perform maintenance tasks and other
> background operations essential to the functioning of the PostgreSQL
> database.
> > They include processes such as:
> >
> > 1. Autovacuum Workers
> > 2. WAL Writer
> > 3. Background Writer
> > 4. Logical Replication Workers
> > 5. Custom Background Workers
> >
> > In the pg_stat_io view, statistics related to I/O operations performed
> by these background workers are recorded.
> >
> > Regards,
> > Imtiaz
> >
> >
> > On Wed, 15 May 2024, 01:26 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
> >
> >
> >
> >
> >
>