Re: Enabling Checksums

On Wed, 2013-04-10 at 20:17 +0100, Simon Riggs wrote:

> OK, so we have a single combined "calculate a checksum for a block"
> function. That uses Jeff's zeroing trick and Ants' bulk-oriented
> performance optimization.
> 
> 
> For buffer checksums we simply calculate for the block.

Sounds good.

> For WAL full page writes, we first set the checksums for all defined
> buffers, then calculate the checksum of remaining data plus the
> pd_checksum field from each block using the normal WAL CRC32.
> 
> Seems good to me. One set of fast code. And it avoids the weirdness
> that the checksum stored on the full page is actually wrong.

Oh, that's a nice benefit.

> It also means that the WAL checksum calculation includes the hole, yet
> we do not include the data for the hole. So we have to do an extra
> copy when restoring the backuo block.

I like this, but it sounds like there is some room for discussion on
some of these points. I assume changes to the WAL checksums are 9.4
material?

I'm satisfied with SIMD data checksums in 9.3 and that we have a plan
for using SIMD for WAL checksums later.

Regards,
	Jeff Davis