Re: Config reload/restart preview

On 20 June 2013 13:16, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com> wrote:

>
>
>
> On Thu, Jun 20, 2013 at 4:34 PM, Thom Brown <thom(at)linux(dot)com> wrote:
>
>> Hi,
>>
>> I've noticed that there's no easy way of checking which settings will
>> change if the config is reloaded, and I think not being able to do this can
>> cause some unfortunate problems.
>>
>> For example, a customer went to change their configuration, just setting
>> log_autovacuum_min_duration to about 20 seconds, and reloaded the server.
>> However, the log file swelled to over 5GB in size before they realised
>> something was wrong, and then reverted the change. It transpired that the
>> reload also pulled in a log_statements change from 'ddl' to 'all' that
>> someone must have changed at some point without applying it.
>>
>> Should we have a way of previewing changes that would be applied if we
>> reloaded/restarted the server?
>>
>> For example:
>>
>> pg_ctl previewconfig
>>
>> SIGHUP: log_statements will change from 'ddl' to 'all'
>> SIGHUP: log_vacuum_min_duration will change from -1 to 20000
>> POSTMASTER: fsync will change from 'on' to 'off'
>>
>> I'm not proposing this specifically, but something that would provide
>> such information.
>>
>>
> May be we can have a nice little utility which can show configuration diff
> between two running servers, or a running server and its modified conf file
> ?
>

Well checking for configuration differences between 2 running servers is
kind of a separate feature, and I'm not sure it's going to be needed by
that many DBAs. If they wanted to see differences, they could just create
a foreign table to the other and do an anti join.

However, comparing the difference between "a running server and its
modified conf file" is the case I'm describing. I'd personally prefer not
to have a whole new utility if it can be avoided. Magnus mentioned making
the necessary changes to pg_ctl so that it can provide unlogged feedback,
and pg_ctl does feel more like the go-to tool for such a feature. Apache
has apache2ctl which is responsible for starting, stopping and restarting
the server, but also provides a configtest option to check that a restart
won't bring the service down. This seems to be a feature in the same kind
of area. In fact that's pretty much the only action apache2ctl does that
pg_ctl doesn't do.

--
Thom