Date: Tue, 14 May 2024 22:26:22 +0200 (CEST)
From: Dimitrios Apostolou <jimis@gmx.net>
To: pgsql-general@lists.postgresql.org
Subject: pg_stat_io clarifications: background worker, writes and reads
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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  | wr=
ites | write_time | writebacks | writeback_time | extends | extend_time | =
op_bytes |   hits    | evictions | reuses | fsyncs | fsync_time |         =
 stats_reset
=2D------------------+---------------+---------+---------+-------------+--=
------+------------+------------+----------------+---------+-------------+=
----------+-----------+-----------+--------+--------+------------+--------=
-----------------------
  background worker | relation      | normal  | 5139575 | 2196288.011 |  6=
3277 |    1766.94 |          0 |              0 |       0 |           0 | =
    8192 | 876913705 |   5139653 |        |      0 |          0 | 2024-04-=
08 08:50:02.971192+00


Thank you in advance,
Dimitris