docs should mention that max_wal_size default depends on WAL segment size

Hi,

while investigating something on a cluster with a non-default WAL
segment (say 256MB), I've noticed a somewhat surprising behavior of
max_wal_size default. While the docs claim the default is 1GB, the
actual default depends on the WAL segment size.

For example with the 256MB WAL segments, you end up with this:

test=# show max_wal_size ;
max_wal_size
--------------
16GB
(1 row)

This behavior is not entirely new - I've noticed it on 10, before the
WAL segment size was moved to initdb (which made it more likely to be
used). It's even more surprising there, because it leaves

#max_wal_size = 1GB

in the sample config, while fc49e24f at least emits the actual value.

But I'd say not mentioning this behavior in the docs is a bug.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services