On 04/22/2013 09:25 AM, Ants Aasma wrote:
> This leaves lingering doubts about the quality of the checksum. It's
> hard if not impossible to prove absence of interesting patterns that
> would trigger collisions. I do know the checksum quality is miles
> ahead of the Fletcher sum originally proposed and during the last week
> I haven't been able to think of a way to make the collision rate
> significantly differ from CRC.

When we originally discussed this feature, we were potentially
discussing a checksum algo which produced collisions for 1 out of 256
pages.  That approach was considered acceptable, since it would be very
unlikely for such a collision to occur across multiple corrupted pages,
and fairly rare to have only one corrupted page.

So my perspective is, if we're doing better than 1 in 256, it's good enough.

Josh Berkus
PostgreSQL Experts Inc.

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