Re: Page Checksums

* Andres Freund (andres@anarazel.de) wrote:
> On Monday, December 19, 2011 03:33:22 PM Alvaro Herrera wrote:
> > > I do like the idea of putting the CRC info in a relation fork, if it can
> > > be made to work decently, as we might be able to then support it on a
> > > per-relation basis, and maybe even avoid the on-disk format change..
> > > 
> > I think the main objection to that idea was that if you lose a single
> > page of CRCs you have hundreds of data pages which no longer have good
> > CRCs.
> Which I find a pretty non-argument because there is lots of SPOF data in a 
> cluster (WAL, control record) anyway...
> If recent data starts to fail you have to restore from backup anyway.

I agree with Andres on this one..  Also, if we use CRC on the pages in
the CRC, hopefully we'd be able to detect when a bad block impacted the
CRC fork and differentiate that from a whole slew of bad blocks in the
heap..

There might be an issue there with handling locking and having to go
through the page-level lock on the CRC, which locks a lot more pages in
the heap and therefore reduces scalability..  Don't we have a similar
issue with the visibility map though?

	Thanks,

		Stephen