On Fri, Apr 12, 2013 at 09:28:42PM +0200, Andres Freund wrote:
> > 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.
>
> 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.
>
> 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?
I thought the idea is that we were going to re-use the already-computed
CRC checksum on the page, and we only have 16-bits of storage for that.
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ It's impossible for everything to be true. +