Re: crash-safe visibility map, take three

On Wed, 2010-12-01 at 15:59 -0500, Robert Haas wrote:
> As for CRCs, there's a pretty direct chain of inference here:
> 
> 1. CRCs are hard (really impossible) because we have hint bits.

I would disagree with "impossible". If we don't set hint bits during
reading; and when we do set them, we log them (including full page
writes); then we can do CRCs.

Those things have costs, but we might be willing to pay them if we had a
bulk loading strategy that avoids or mitigates the costs.

The reason we can't do CRCs now is because hint bits violate the
WAL-before-data rule; not because of hint bits themselves. We're talking
about adding another feature that breaks the rule, in a more complex way
than hint bits.

I just wanted to step back for a second and consider the problem from a
different angle before we committed to that.

Regards,Jeff Davis