Re: Enabling Checksums

On Thu, 2013-04-11 at 20:12 +0100, Simon Riggs wrote:

> So, if we apply a patch like the one attached, we then end up with the
> WAL checksum using the page checksum as an integral part of its
> calculation. (There is no increase in code inside WALInsertLock,
> nothing at all touched in that area).
> 
> 
> Then all we need to do is make PageSetChecksumInplace() use Ants' algo
> and we're done.
> 
> 
> Only point worth discussing is that this change would make backup
> blocks be covered by a 16-bit checksum, not the CRC-32 it is now. i.e.
> the record header is covered by a CRC32 but the backup blocks only by
> 16-bit. 

FWIW, that's fine with me. 

> (Attached patch is discussion only. Checking checksum in recovery
> isn't coded at all.)

I like it.

A few points:

* Given that setting the checksum is unconditional in a backup block, do
we want to zero the checksum field when the backup block is restored if
checksums are disabled? Otherwise we would have a strange situation
where some blocks have a checksum on disk even when checksums are
disabled.

* When we do PageSetChecksumInplace(), we need to be 100% sure that the
hole is empty; otherwise the checksum will fail when we re-expand it. It
might be worth a memset beforehand just to be sure.

Regards,
	Jeff Davis