Re: time-delayed standbys

On Fri, Jul 1, 2011 at 3:25 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Jun 30, 2011 at 1:51 PM, Kevin Grittner
> <Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
>> I think doing anything in PostgreSQL around this beyond allowing
>> DBAs to trust their server clocks is insane.  The arguments for
>> using and trusting ntpd is pretty much identical to the arguments
>> for using and trusting the OS file systems.
>
> Except that implementing our own file system would likely have more
> benefit and be less work than implementing our own time
> synchronization, at least if we want it to be reliable.
>
> Again, I am not trying to pretend that this is any great shakes.
> MySQL's version of this feature apparently does somehow compensate for
> time skew, which I assume must mean that their replication works
> differently than ours - inter alia, it probably requires a TCP socket
> connection between the servers.  Since we don't require that, it
> limits our options in this area, but also gives us more options in
> other areas.  Still, if I could think of a way to do this that didn't
> depend on time synchronization, then I'd be in favor of eliminating
> that requirement.  I just can't; and I'm inclined to think it isn't
> possible.
>
> I wouldn't be opposed to having an option to try to detect time skew
> between the master and the slave and, say, display that information in
> pg_stat_replication.  It might be useful to have that data for
> monitoring purposes, and it probably wouldn't even be that much code.
> However, I'd be a bit hesitant to use that data to "correct" the
> amount of time we spend waiting for time-delayed replication, because
> it would doubtless be extremely imprecise compared to real time
> synchronization, and considerably more error-prone.  IOW, what you
> said.

I agree with Robert. It's difficult to implement time-synchronization feature
which can deal with all the cases, and I'm not sure if that's really
worth taking
our time.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center