confusing checkpoint_flush_after / bgwriter_flush_after

Hi,

while doing some benchmarking, I've once again got confused by the 
default settings for checkpoint_flush_after and bgwriter_flush_after. 
The sample config says this:

#checkpoint_flush_after = 0   # 0 disables,                              # default is 256kB on linux, 0 otherwise

#bgwriter_flush_after = 0     # 0 disables,                              # default is 512kB on linux, 0 otherwise

I find this pretty confusing, because for all other GUCs in the file, 
the commented-out value is the default one. In this case that would mean 
"0", disabling the flushing.

But in practice we use platform-dependent defaults - 256/512K on Linux, 
0 otherwise. There are other GUCs where the default is 
platform-specific, but none of them suggests "disabled" is the default 
state.

While the 9.6 cat is out of the bag, I think we can fix this quite 
easily - use "-1" to specify the default value should be used, and use 
that in the sample file. This won't break any user configuration.

If that's considered not acceptable, perhaps we should at least improve 
the comments, so make this clearer.

regards

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