Re: 16-bit page checksums for 9.2

On Thu, Dec 29, 2011 at 6:44 PM, Kevin Grittner
<Kevin.Grittner@wicourts.gov> wrote:
> positives.  To get this right for a checksum in the page header,
> double-write would need to be used for all cases where
> full_page_writes now are used (i.e., the first write of a page after
> a checkpoint), and for all unlogged writes (e.g., hint-bit-only
> writes).  There would be no correctness problem for always using
> double-write, but it would be unnecessary overhead for other page
> writes, which I think we can avoid.

Unless I'm missing something, double-writes are needed for all writes,
not only the first page after a checkpoint. Consider this sequence of
events:

1. Checkpoint
2. Double-write of page A (DW buffer write, sync, heap write)
3. Sync of heap, releasing DW buffer for new writes.... some time goes by
4. Regular write of page A
5. OS writes one part of page A
6. Crash!

Now recovery comes along, page A is broken in the heap with no
double-write buffer backup nor anything to recover it by in the WAL.

--
Ants Aasma