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Re: how is pitr replay interruption time determined?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Treat <xzilla(at)users(dot)sourceforge(dot)net>
Cc: pgsql-admin(at)postgresql(dot)org, Simon Riggs <simon(at)2ndquadrant(dot)com>
Subject: Re: how is pitr replay interruption time determined?
Date: 2007-08-28 21:59:20
Message-ID: 4555.1188338360@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-admin
Robert Treat <xzilla(at)users(dot)sourceforge(dot)net> writes:
> Is there some way to force checkpoints on a db doing wal replay? 

No, it's hardwired to do it when it sees a checkpoint record in the WAL stream.

> pg_control last modified:             Mon Aug 27 12:12:55 2007
> Time of latest checkpoint:            Mon Jul 30 19:17:37 2007

After looking again at the code, the "last modified" time is the time
that a recovery checkpoint was last done, and the "latest checkpoint"
is the timestamp of the WAL-stream checkpoint record that triggered it.
In a situation where you're catching up on historical WAL they could be
far apart, but when a slave is just following the master there shouldn't
be a huge difference --- not more than the maximum time to fill a WAL
record and ship it over to the slave, for sure.

(BTW, I misread it before --- it looks like the "at log time" value
printed at startup *is* taken from the checkpoint record that it's
trying to roll forward from.)

Assuming that you're absorbing data from the master at a steady rate,
the only reason I can see for the timestamps to be so old is if the
"rm_safe_restartpoint" checks are always failing.  I seem to remember
that we found and fixed a bug that could cause something like that,
but I can't find any trace of it in the CVS logs.  Simon, do you
recall such a problem post-8.2.0?

			regards, tom lane

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