Re: [HACKERS] Measuring replay lag

On Wed, Jan 4, 2017 at 12:22 PM, Thomas Munro
<thomas.munro@enterprisedb.com> wrote:
> (replay_lag - (write_lag / 2) may be a cheap proxy
> for a lag time that doesn't include the return network leg, and still
> doesn't introduce clock difference error)

(Upon reflection it's a terrible proxy for that because of the mix of
write/flush work done by WAL receiver today, but would improve
dramatically if the WAL writer were doing the flushing.  A better yet
proxy might involve also tracking receive_lag which doesn't include
the write() syscall.  My real point is that there are ways to work
backwards from the two-way round trip time to get other estimates, but
no good ways to undo the damage that would be done to the data if we
started using two systems' clocks.)

-- 
Thomas Munro
http://www.enterprisedb.com