Re: CRCs

On Fri, Jan 12, 2001 at 04:10:36PM -0800, Alfred Perlstein wrote:
> Nathan Myers <ncm@zembu.com> [010112 15:49] wrote:
> >
> > Obviously it's better to configure the disk so that it doesn't
> > lie about what's been written.
> 
> I thought WAL+fsync wasn't supposed to allow this to happen?

It's an OS and hardware configuration matter; you only get correct
WAL+fsync semantics if the underlying system is configured right.  
IDE disks are almost always configured wrong, to spoof benchmarks; 
SCSI disks sometimes are.

If they're configured wrong, then (now that we have a CRC in the 
log entry) in the event of a power outage the database might come 
back with recently-acknowledged transaction results discarded.
That's a lot better than a corrupt database, but it's not 
industrial-grade semantics.  (Use a UPS.)

Nathan Myers
ncm@zembu.com