To: Robert Haas <robertmhaas@gmail.com>
cc: Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
Subject: Re: time-delayed standbys 
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Wed, 20 Apr 2011 11:05:43 -0400"
Date: Wed, 20 Apr 2011 11:15:06 -0400
Message-ID: <27149.1303312506@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Robert Haas <robertmhaas@gmail.com> writes:
> I am a bit concerned about the reliability of this approach.  If there
> is some network lag, or some lag in processing from the master, we
> could easily get the idea that there is time skew between the machines
> when there really isn't.  And our perception of the time skew could
> easily bounce around from message to message, as the lag varies.  I
> think it would be tremendously ironic of the two machines were
> actually synchronized to the microsecond, but by trying to be clever
> about it we managed to make the lag-time accurate only to within
> several seconds.

Well, if walreceiver concludes that there is no more than a few seconds'
difference between the clocks, it'd probably be OK to take the master
timestamps at face value.  The problem comes when the skew gets large
(compared to the configured time delay, I guess).

			regards, tom lane