Understanding “max_wal_size” and “min_wal_size” parameters default values from postgresql.conf file
-
04-10-2020 - |
Question
Default values are, according to documentation for min_wal_size
and max_wal_size
parameters:
For max_wal_size
: The default is 1 GB
For min_wal_size
: The default is 80 MB
Then I look this parameters from my database config:
select name, setting, unit
from pg_settings
where name in ('min_wal_size', 'max_wal_size')
Gives result:
name | setting | unit
----------------------------------
max_wal_size | 64 |
min_wal_size | 5 |
I have 2 questions:
1) Why these values doesn't match default values, which are shown in docs? I never changed config settings at all.
2) Why unit
column is empty/NULL for these parameters? What means 64 and 5 values in this case? MB
? GB
? Or what?
Why this is not like for example work_mem
parameter, when everything is clear:
name | setting | unit
----------------------------------
work_mem | 4096 | kB
Solution
These are 16 MB WAL segments by default. The manual:
The system physically divides this sequence into WAL segment files, which are normally 16MB apiece (although the segment size can be altered when building PostgreSQL)
So, it's just default values as advertised:
select name, setting, setting::int * 16 || 'MB' AS setting_in_mb
from pg_settings
where name in ('min_wal_size', 'max_wal_size');
unit
in pg_settings
is present when it's an actual base unit like second or MB. In this case the "unit" would be "WAL segments", which can be configured before compiling Postgres. The unit
column is just NULL for those.