Date: Wed, 4 Nov 2015 11:33:51 +0100
From: Andres Freund <andres@anarazel.de>
To: Michael Paquier <michael.paquier@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Jeff Janes <jeff.janes@gmail.com>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Re: BUG #13685: Archiving while idle every
 archive_timeout with wal_level hot_standby
Message-ID: <20151104103351.GB29170@awork2.anarazel.de>
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On 2015-11-04 16:01:28 +0900, Michael Paquier wrote:
> On Wed, Nov 4, 2015 at 8:39 AM, Andres Freund <andres@anarazel.de> wrote:
> > On November 4, 2015 12:37:02 AM GMT+01:00, Michael Paquier wrote:
> >>On a completely idle system, I don't think we should log any standby
> >>records. This is what ~9.3 does.
> >
> > Are you sure? I think it'll around checkpoints, no? I thought Heikki had fixed that, but looking sound that doesn't seem to be the case.
> 
> Er, yes, sorry. I should have used clearer words: I meant idle system
> with something running nothing including internal checkpoints.

Uh, but you'll always have checkpoints happen on wal_level =
hot_standby, even in 9.3?  Maybe I'm not parsing your sentence right.

As soon as a single checkpoint ever happened the early-return logic in
CreateCheckPoint() will fail to take the LogStandbySnapshot() in
CreateCheckPoint() into account. The test is:
    if (curInsert == ControlFile->checkPoint +
	MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
	ControlFile->checkPoint == ControlFile->checkPointCopy.redo)
which obviously doesn't work if there's been a WAL record logged after
the redo pointer has been determined etc.

The reason that a single checkpoint is needed to "jumpstart" the
pointless checkpoints is that otherwise we'll never have issued a
LogStandbySnapshot() and thus the above code block works if we started
from a proper shutdown checkpoint.


Independent of the idle issue, it seems to me that the location of the
LogStandbySnapshot() is actually rather suboptimal - it really should
really be before the CheckPointGuts(), not afterwards. As closer it's to
the redo pointer of the checkpoint a hot standby node starts up from,
the sooner that node can reach consistency.  There's no difference for
the first time a node starts from a basebackup (since we gotta replay
that checkpoint anyway before we're consistent), but if we start from a
restartpoint...

Greetings,

Andres Freund


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