Re: Enabling Checksums

On Fri, 2013-04-12 at 21:28 +0200, Andres Freund wrote:
> That means we will have to do the verification for this in
> ValidXLogRecord() *not* in RestoreBkpBlock or somesuch. Otherwise we
> won't always recognize the end of WAL correctly.
> And I am a bit wary of reducing the likelihood of noticing the proper
> end-of-recovery by reducing the crc width.

Good point.

> Why again are we doing this now? Just to reduce the overhead of CRC
> computation for full page writes? Or are we forseeing issues with the
> page checksums being wrong because of non-zero data in the hole being
> zero after the restore from bkp blocks?

That shouldn't be a problem, because the block is not expected to have a
proper checksum in WAL, and it will be recalculated before being
written. So I see these changes as mostly independent.

The reason we're discussing right now is because, when choosing the
checksum algorithm, I was hoping that it might be usable in the future
for WAL backup blocks. I'm convinced that they can be; and the primary
question now seems to be "should they be", which does not need to be
settled right now in my opinion.

Anyway, I would be perfectly happy if we just got the SIMD algorithm in
for data pages. The support for changing the WAL checksums seems
lukewarm, and there might be quite a few alternatives (e.g. optimizing
the CRC for backup blocks as Heikki suggested) to achieve that
performance goal.

Regards,
Jeff Davis