Re: CRCs

On Fri, Jan 12, 2001 at 12:35:14PM -0800, Nathan Myers wrote:
> Vadim wrote:
> > What do you mean by "external agents"?
> 
> External agents include RAM bit drops and noise on cables when
> blocks are (read and re-) written.  Every time data is moved, 
> there is a chance of an undetected error being introduced.  The 
> disk only promises (within limits) to deliver the sector that 
> was written; it doesn't promise that what was written is what 
> you meant to write.  Errors of this sort accumulate unless 
> caught by end-to-end checks.
> 
> External agents include bugs in database code, bugs in OS code,
> bugs in disk controller firmware, and bugs in disk firmware.
> Each can result in clobbered data, blocks being written in the
> wrong place, blocks said to be written but not, and any number
> of other variations.  All this code is written by humans, and
> even the most thorough testing cannot cover even the majority
> of code paths.
> 
> External agents include sector errors not caught by the disk CRC: 
> the disk only promises to keep the number of errors delivered to a
> reasonably low (and documented) level.  It's up to the user to 
> notice the errors that slip through.

Interestingly, right after I posted this I noticed that cron 
noticed a corrupt inode in /dev on my machine.  The disk is 
happy with it, but I'm not...

Nathan Myers
ncm@zembu.com