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Re: Extra XLOG in Checkpoint for StandbySnapshot

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Amit Kapila <amit(dot)kapila(at)huawei(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Extra XLOG in Checkpoint for StandbySnapshot
Date: 2013-01-07 13:44:41
Message-ID: CA+U5nMK_5LAvqQRnwj6Q_4Oo52jCSkR=CZ_JS4=AJaRgvaCOmw@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 7 January 2013 13:33, Amit Kapila <amit(dot)kapila(at)huawei(dot)com> wrote:

>> If we skip the WAL record in the way you suggest, we'd be unable to
>> start quickly in some cases.
>
> If there are any operations happened which have generated WAL, then on next
> checkpoint interval the checkpoint operation should happen.
> Which cases will it not able to start quickly?

The case where we do lots of work but momentarily we weren't doing
anything when we took the snapshot.

The absence of write transactions at one specific moment gives no
indication of behaviour at other points across the whole checkpoint
period.

If you make the correct test, I'd be more inclined to accept the premise.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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