Re: How much do the hint bits help?

On Wed, Dec 22, 2010 at 9:52 AM, Simon Riggs <simon@2ndquadrant.com> wrote:

> So what you suggest works only if we restrict CRC checking to blocks
> incoming to the buffer cache, but leaves us unable to do CRC checks on
> blocks once in the buffer cache. Since many blocks stay in cache almost
> constantly, we're left with the situation that the most heavily used
> parts of the database seldom get CRC checked.

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.

I believe that most of the people talking about and wanting checksums
so far have been wanting them to verify I/O, not to verify that PG has
no bugs, that RAM is staying charged correctly, and that no stray bits
have been flipped, and that nobody else happens to be scribbling over
our shared buffers.

Being able to arbitrary (i.e at any point in time) prove that the
shared buffers contents are exactly what they should be may be a
worthy goal, but that's many orders of magnitude more difficult than
verifying that the bytes we read from disk are the ones we wrote to
disk.

a.



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