Re: [HACKERS] Reducing pg_ctl's reaction time

Hi,

On 2017-06-26 16:49:07 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > Arguably we could and should improve the logic when the server has
> > started, right now it's pretty messy because we never treat a standby as
> > up if hot_standby is disabled...
> 
> True.  If you could tell the difference between "HS disabled" and "HS not
> enabled yet" from pg_control, that would make pg_ctl's behavior with
> cold-standby servers much cleaner.  Maybe it *is* worth messing with the
> contents of pg_control at this late hour.

I'm +0.5.


> My inclination for the least invasive fix is to leave the DBState
> enum alone and add a separate hot-standby state field with three
> values (disabled/not-yet-enabled/enabled).

Yea, that seems sane.


> Then pg_ctl would start
> probing the postmaster when it saw either DB_IN_PRODUCTION DBstate
> or hot-standby-enabled.  (It'd almost not have to probe the postmaster
> at all, except there's a race condition that the startup process
> will probably change the field a little before the postmaster gets
> the word to open the gates.)  On the other hand, if it saw
> DB_IN_ARCHIVE_RECOVERY with hot standby disabled, it'd stop waiting.

It'd be quite possible to address the race-condition by moving the
updating of the control file to postmaster, to the
CheckPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY) block. That'd require
updating the control file from postmaster, which'd be somewhat ugly.
Perhaps that indicates that field shouldn't be in pg_control, but in the
pid file?

Greetings,

Andres Freund