Re: Page Checksums

On Sun, Dec 18, 2011 at 7:51 PM, Jesper Krogh <jesper(at)krogh(dot)cc> wrote:
> I dont know if it would be seen as a "half baked feature".. or similar,
> and I dont know if the hint bit problem is solvable at all, but I could
> easily imagine checksumming just "skipping" the hit bit entirely.

That was one approach discussed. The problem is that the hint bits are
currently in each heap tuple header which means the checksum code
would have to know a fair bit about the structure of the page format.
Also the closer people looked the more hint bits kept turning up
because the coding pattern had been copied to other places (the page
header has one, and index pointers have a hint bit indicating that the
target tuple is deleted, etc). And to make matters worse skipping
individual bits in varying places quickly becomes a big consumer of
cpu time since it means injecting logic into each iteration of the
checksum loop to mask out the bits.

So the general feeling was that we should move all the hint bits to a
dedicated part of the buffer so that they could all be skipped in a
simple way that doesn't depend on understanding the whole structure of
the page. That's not conceptually hard, it's just a fair amount of
work. I think that's where it was left off.

There is another way to look at this problem. Perhaps it's worth
having a checksum *even if* there are ways for the checksum to be
spuriously wrong. Obviously having an invalid checksum can't be a
fatal error then but it might still be useful information. Rright now
people don't really know if their system can experience torn pages or
not and having some way of detecting them could be useful. And if you
have other unexplained symptoms then having checksum errors might be
enough evidence that the investigation should start with the hardware
and get the sysadmin looking at hardware logs and running memtest
sooner.

--
greg