Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

At Sat, 04 Mar 2023 18:21:09 -0500, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote in
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > Just pushed the actual pg_stat_io view, the splitting of the tablespace test,
> > and the pg_stat_io tests.
>
> One of the test cases is flapping a bit:
>
> diff -U3 /home/pg/build-farm-15/buildroot/HEAD/pgsql.build/src/test/regress/expected/stats.out /home/pg/build-farm-15/buildroot/HEAD/pgsql.build/src/test/regress/results/stats.out
> --- /home/pg/build-farm-15/buildroot/HEAD/pgsql.build/src/test/regress/expected/stats.out 2023-03-04 21:30:05.891579466 +0100
> +++ /home/pg/build-farm-15/buildroot/HEAD/pgsql.build/src/test/regress/results/stats.out 2023-03-04 21:34:26.745552661 +0100
> @@ -1201,7 +1201,7 @@
> SELECT :io_sum_shared_after_reads > :io_sum_shared_before_reads;
> ?column?
> ----------
> - t
> + f
> (1 row)
>
> DROP TABLE test_io_shared;
>
> There are two instances of this today [1][2], and I've seen it before
> but failed to note down where.

The concurrent autoanalyze below is logged as performing at least one
page read from the table. It is unclear, however, how that analyze
operation resulted in 19 hits and 2 reads on the (I think) single-page
relation.

In any case, I think we need to avoid such concurrent autovacuum/analyze.

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=grison&dt=2023-03-04%2021%3A19%3A39

2023-03-04 22:36:27.781 CET [4073:106] pg_regress/stats LOG: statement: ALTER TABLE test_io_shared SET TABLESPACE regress_tblspace;
2023-03-04 22:36:27.838 CET [4073:107] pg_regress/stats LOG: statement: SELECT COUNT(*) FROM test_io_shared;
2023-03-04 22:36:27.864 CET [4255:5] LOG: automatic analyze of table "regression.public.test_io_shared"
avg read rate: 5.208 MB/s, avg write rate: 5.208 MB/s
buffer usage: 17 hits, 2 misses, 2 dirtied
2023-03-04 22:36:28.024 CET [4073:108] pg_regress/stats LOG: statement: SELECT pg_stat_force_next_flush();
2023-03-04 22:36:28.024 CET [4073:108] pg_regress/stats LOG: statement: SELECT pg_stat_force_next_flush();
2023-03-04 22:36:28.027 CET [4073:109] pg_regress/stats LOG: statement: SELECT sum(reads) AS io_sum_shared_after_reads
FROM pg_stat_io WHERE io_context = 'normal' AND io_object = 'relation'

> [1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=grison&dt=2023-03-04%2021%3A19%3A39
> [2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mule&dt=2023-03-04%2020%3A30%3A05

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center