Re: time-delayed standbys

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.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company