On Tuesday 01 December 2009 14:38:26 marcin mank wrote:
> On Mon, Nov 30, 2009 at 9:27 PM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> > Simon Riggs wrote:
> >> Proposal
> >> * We reserve enough space on a disk block for a CRC check. When a dirty
> >> block is written to disk we calculate and annotate the CRC value, though
> >> this is *not* WAL logged.
> > Imagine this:
> > 1. A hint bit is set. It is not WAL-logged, but the page is dirtied.
> > 2. The buffer is flushed out of the buffer cache to the OS. A new CRC is
> > calculated and stored on the page.
> > 3. Half of the page is flushed to disk (aka torn page problem). The CRC
> > made it to disk but the flipped hint bit didn't.
> > You now have a page with incorrect CRC on disk.
> What if we treated the hint bits as all-zeros for the purpose of CRC
> calculation? This would exclude them from the checksum.
That sounds like doing a complete copy of the wal page zeroing specific fields
and then doing wal - rather expensive I would say. Both, during computing the
checksum and checking it...
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