On 2013-04-12 15:31:36 -0400, Bruce Momjian wrote:
> 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.

Well, but the proposal seems to be to do this also for non-checksum
enabled datadirs, so ...


Andres Freund

 Andres Freund                     http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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