Hi,
I am trying to understand the evictions statistic in pg_stat_io. I know what
evictions are, so this is not the question.
Given this:
postgres=# select version();
version
----------------------------------------------------------------------------------------------
PostgreSQL 17devel on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14)
12.2.0, 64-bit
(1 row)
postgres=# select pg_stat_reset_shared('io');
pg_stat_reset_shared
----------------------
(1 row)
postgres=# create table t ( a int, b text, c text, d text );
CREATE TABLE
postgres=# insert into t select i, md5(i::text), md5(i::text), md5(i::text)
from generate_series(1,1000000) i;
INSERT 0 1000000
postgres=# select backend_type,evictions,context
from pg_stat_io
where backend_type = 'client backend'
and object ='relation';
backend_type | evictions | context
----------------+-----------+-----------
client backend | 0 | bulkread
client backend | 0 | bulkwrite
client backend | 207 | normal
client backend | 0 | vacuum
Shouldn't these evictions show up under context blkwrite? The description in
docs is:
"Number of times a block has been written out from a shared or local buffer in
order to make it available for another use.
In context normal, this counts the number of times a block was evicted from a
buffer and replaced with another block. In contexts bulkwrite, bulkread, and
vacuum, this counts the number of times a block was evicted from shared buffers
in order to add the shared buffer to a separate, size-limited ring buffer for
use in a bulk I/O operation."
As far as I understand this, a ring buffer is used in this case. Do I miss
something?
Many thanks in advance
Daniel
