Re: WAL format

On Monday 07 December 2009 21:44:37 Tom Lane wrote:
> "Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> > Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> wrote:
> >> In particular I wonder why we bother with the page headers.
> >
> > Since we re-use the file for a new segment, without overwriting the
> > old contents, it seems like we would need to do *something* to
> > reliably determine when we've hit the end of a segment and have
> > moved into old data from a previous use of the file.  Would your
> > proposed changes cover that adequately?
> AFAICT the proposal would make us 100% dependent on the record CRC
> to detect when a record has been torn (ie, only the first few sectors
> made it to disk).  I'm a bit nervous about that from a reliability
> standpoint --- with a 32-bit CRC you've got a 1-in-4-billion chance
> of accepting bad data.  Checking the page headers too gives us many
> more bits that have to be as-expected to consider the data good.
One could argue that thats a good argument to go back to 64bit CRCs. 
Considering that they are more seldomly computed with such a change and that 
CPUs got more modern...

Andres