Re: Patch for fail-back without fresh backup

Bruce Momjian escribió:
> On Thu, Oct 24, 2013 at 11:14:14PM +0300, Heikki Linnakangas wrote:
> > On 24.10.2013 23:07, Josh Berkus wrote:

> > >What kind of overhead are we talking about here?
> >
> > 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 don't think so. There's extra WAL written, but there's no
flush-and-wait until end of transaction (as has always been).

> I thought that was the down-side to WAL logging hint bits, not the WAL
> volume itself.

I don't think this is true either.

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