Re: How much do the hint bits help?

On Wed, 2010-12-22 at 10:45 -0500, Tom Lane wrote:
> Aidan Van Dyk  writes:
> > With this statement, you just moved the goal posts on the checksumming
> > ideas.  In fact, you didn't just move the goal posts, you picked the
> > ball up and teleported it to another stadium.
> 
> What he said.  I can't imagine that anyone will be interested in any
> case other than "set the CRC immediately before writing, and check it
> upon first reading the page in".  Maintaining it continuously while the
> page is in shared memory is completely insane from a cost-versus-benefit
> perspective.

If you insist on setting hint-bits, then that is probably true.

Many people experience almost no I/O these days, and there's a strong
correlation between people caring about their data and also being
willing to spend big $s on cache. We need to protect our users, however
much money they spent on cache; I would argue the more money they spent
on cache the harder we should be trying to protect them.

I'm sure it will take a little while for everybody to understand why a
full CRC implementation is both necessary and now possible. Paradigm
shifts of thought do seem like teleports, but they can be beneficial.

-- Simon Riggs           http://www.2ndQuadrant.com/books/PostgreSQL Development, 24x7 Support, Training and Services