Re: Fast promotion, loose ends

On 22.04.2013 10:58, Simon Riggs wrote:
> On 22 April 2013 08:13, Heikki Linnakangas<hlinnakangas@vmware.com>  wrote:
>> 2. There is no way to perform 'fast promotion' using the trigger file. That
>> feature is only available using "pg_ctl promote". When "pg_ctl promote" was
>> introduced, it was not meant to replace the trigger file method, as that is
>> also very useful in many situations. (as explained here:
>> http://www.postgresql.org/message-id/5112A54B.8090500@vmware.com)
>
> I realise that it *is* possible to trigger fast promotion using a
> file, since that is exactly the method we use to implement the feature
> by pg_ctl.
>
> In fact, pg_ctl promote is just a nice paint job over the trigger file concept.
>
> So, to initiate promotion, you can create a file called
> $DATADIR/fast_promote or $DATADIR/promote
>
> Does that solve the issue?

Hmm. That requires write access to $DATADIR, so that's not quite the 
same thing as the trigger_file recovery.conf option. But if you really 
want to keep the 'slow' option, we could add an option to recovery.conf 
for it. I'm thinking something like this:

trigger_file = '/mnt/foo/trigger'    # file to watch for
trigger_file_method = 'slow'         # slow / fast, 'fast' is default.

You would have to decide which method to use when you create 
recovery.conf file, rather than when you promote, but I think that would OK.

>> Putting points 1 and 2 together, I think the best solution is to always
>> perform 'fast' promotion when in standby mode, and remove pg_ctl -m
>> fast/smart option altogether. There is no need to expose that to users.
>
> We can make pg_ctl promote write the fast_promote file by default and
> remove the command line option altogether.

Sounds good to me.

> I would like to retain the option to promote with a full checkpoint,
> for safety reasons, so continue to use both fast_promote and promote
> files under the covers.

Ok. As long as 'fast' is the default and you get the same functionality 
with the trigger file method and pg_ctl promote, that works for me.

Thanks!

- Heikki