Re: comment for "fast promote"

On Fri, Jul 26, 2013 at 11:19 AM, Tomonari Katsumata
<katsumata(dot)tomonari(at)po(dot)ntts(dot)co(dot)jp> wrote:
> Hi Fujii-san,
>
> Thank you for response.
>
>
> (2013/07/25 21:15), Fujii Masao wrote:
>> On Thu, Jul 25, 2013 at 5:33 PM, Tomonari Katsumata
>> <katsumata(dot)tomonari(at)po(dot)ntts(dot)co(dot)jp> wrote:
>>> Hi,
>>>
>>> Now I'm seeing xlog.c in 93_stable for studying "fast promote",
>>> and I have a question.
>>>
>>> I think it has an extra unlink command for "promote" file.
>>> (on 9937 line)
>>> -------
>>> 9934 if (stat(FAST_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
>>> 9935 {
>>> 9936 unlink(FAST_PROMOTE_SIGNAL_FILE);
>>> 9937 unlink(PROMOTE_SIGNAL_FILE);
>>> 9938 fast_promote = true;
>>> 9939 }
>>> -------
>>>
>>> Is this command necesary ?
>>
>> Yes, it prevents PROMOTE_SIGNAL_FILE from remaining even if
>> both promote files exist.
>>
> The command("unlink(PROMOTE_SIGNAL_FILE)") here is for
> unusualy case.
> Because the case is when done both procedures below.
> - user create "promote" file on PGDATA
> - user issue "pg_ctl promote"
>
> I understand the reason.
> But I think it's better to unlink(PROMOTE_SIGNAL_FILE) before
> unlink(FAST_PROMOTE_SIGNAL_FILE).
> Because FAST_PROMOTE_SIGNAL_FILE is definetly there but
> PROMOTE_SIGNAL_FILE is sometimes there or not there.

I could not understand why that's better. Could you elaborate that?

> And I have another question linking this behavior.
> I think TriggerFile should be removed too.
> This is corner-case but it will happen.
> How do you think of it ?

I don't have strong opinion about that. I've never heard the complaint
about that current behavior so far.

>> One question is that: we really still need to support normal promote?
>> pg_ctl promote provides only way to do fast promotion. If we want to
>> do normal promotion, we need to create PROMOTE_SIGNAL_FILE
>> and send the SIGUSR1 signal to postmaster by hand. This seems messy.
>>
>> I think that we should remove normal promotion at all, or change
>> pg_ctl promote so that provides also the way to do normal promotion.
>>
> I think he merit of "fast promote" is
> - allowing quick connection by skipping checkpoint
> and its demerit is
> - taking little bit longer when crash-recovery
>
> If it is seldom to happen its crash soon after promoting
> and "fast promte" never breaks consistency of database cluster,
> I think we don't need normal promotion.

You can execute checkpoint after fast promotion for that.

Regards,

--
Fujii Masao