> > But physical log recovery will rewrite every page that was changed
> > after last checkpoint, thus this is not an issue anymore.
> 
> No.  That assumes that when the drive _says_ the block is written, 
> it is really on the disk.  That is not true for IDE drives.  It is 
> true for SCSI drives only when the SCSI spec is implemented correctly,
> but implementing the spec correctly interferes with favorable 
> benchmark results.

You know - this is *core* assumption. If drive lies about this then
*nothing* will help you. Do you remember core rule of WAL?
"Changes must be logged *before* changed data pages written".
If this rule will be broken then data files will be inconsistent
after crash recovery and you will not notice this, w/wo CRC in
data blocks.

I agreed that CRCs could help to detect other errors but probably
it's too late for 7.1

Vadim

Fri, 12 Jan 2001 13:07:56 -0800
"Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
RE: CRCs 
On Fri, Jan 12, 2001 at 01:07:56PM -0800, Mikheev, Vadim wrote:
> > > But physical log recovery will rewrite every page that was changed
> > > after last checkpoint, thus this is not an issue anymore.
> > 
> > No.  That assumes that when the drive _says_ the block is written, 
> > it is really on the disk.  That is not true for IDE drives.  It is 
> > true for SCSI drives only when the SCSI spec is implemented correctly,
> > but implementing the spec correctly interferes with favorable 
> > benchmark results.
> 
> You know - this is *core* assumption. If drive lies about this then
> *nothing* will help you. Do you remember core rule of WAL?
> "Changes must be logged *before* changed data pages written".
> If this rule will be broken then data files will be inconsistent
> after crash recovery and you will not notice this, w/wo CRC in
> data blocks.

You can include the data blocks' CRCs in the log entries.

> I agreed that CRCs could help to detect other errors but probably
> it's too late for 7.1.

7.2 is not too far off.  I'm hoping to see it then.

Nathan Myers
ncm@zembu.com

Fri, 12 Jan 2001 13:49:35 -0800
ncm@zembu.com (Nathan Myers)
Re: CRCs