Re: confusing checkpoint_flush_after / bgwriter_flush_after

On 11/25/2016 01:20 PM, Fabien COELHO wrote:
>
> Hello Tomas,
>
>> #checkpoint_flush_after = 0   # 0 disables,
>>                              # default is 256kB 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.
>
> Although I understand the issue, I'm not sure about -1 as a special
> value to mean the default.
>

Why? We use wal_buffers=-1 to use the default (depending on the size of 
shared_buffers), for example.

>> If that's considered not acceptable, perhaps we should at least
>> improve the comments, so make this clearer.
>
> Yep, what about not putting a value and inverting/adapting the comments,
> maybe something like:
>
>  #checkpoint_flush_after = ...  # default is 256kB on linux, 0 otherwise
>                                 # where 0 disables flushing
>

Yeah, something like that.

regards

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