Re: Patch for fail-back without fresh backup

On Thu, Oct 24, 2013 at 11:14:14PM +0300, Heikki Linnakangas wrote:
> On 24.10.2013 23:07, Josh Berkus wrote:
> >On 10/24/2013 11:12 AM, Heikki Linnakangas wrote:
> >>On 24.10.2013 20:39, Josh Berkus wrote:
> >>>On 10/24/2013 04:15 AM, Pavan Deolasee wrote:
> >>>>If we do what you are suggesting, it seems like a single line patch
> >>>>to me.
> >>>>In XLogSaveBufferForHint(), we probably need to look at this
> >>>>additional GUC
> >>>>to decide whether or not to backup the block.
> >>>
> >>>Wait, what? Why are we having an additional GUC?
> >>>
> >>>I'm opposed to the idea of having a GUC to enable failback. When would
> >>>anyone using replication ever want to disable that?
> >>
> >>For example, if you're not replicating for high availability purposes,
> >>but to keep a reporting standby up-to-date.
> >
> >What kind of overhead are we talking about here? You probably said, but
> >I've had a mail client meltdown and lost a lot of my -hackers emails.
>
> One extra WAL record whenever a hint bit is set on a page, for the
> first time after a checkpoint. In other words, a WAL record needs to
> be written in the same circumstances as with page checksums, but the
> WAL records are much smaller as they don't need to contain a full
> page image, just the block number of the changed block.
>
> Or maybe we'll write the full page image after all, like with page
> checksums, just without calculating the checksums. It might be
> tricky to skip the full-page image, because then a subsequent change
> of the page (which isn't just a hint-bit update) needs to somehow
> know it needs to take a full page image even though a WAL record for
> it was already written.

Sorry to be replying late to this, but while I am not worried about the
additional WAL volume, does this change require the transaction to now
wait for a WAL sync to disk before continuing? I thought that was the
down-side to WAL logging hint bits, not the WAL volume itself.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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