Re: Distributed spares

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Tue, Oct 14, 2008 at 06:12:29AM -0400, Martin K. Petersen wrote:
> >>>>> "Keld" == Keld Jørn Simonsen <keld@dkuug.dk> writes:
> 
> Keld> I have also been thinking a little on this. My idea is that if
> Keld> bit errors develop on disks, then there is first maybe one bit
> Keld> error, and the crc check on the disk sectors then finds and
> Keld> corrects these.
> 
> Keld> If you rewrite such bit errors, then that bit error will be
> Keld> corrected, and you prevent the one-bit error from developing to
> Keld> a two-bit error that is not correctable by the CRC.
> 
> I think you are assuming that disks are much simpler than they
> actually are.
> 
> A modern disk drive protects a 512-byte sector with a pretty strong
> ECC that's capable of correcting errors up to ~50 bytes.  Yes, that's
> bytes.
> 
> Also, many drive firmwares will internally keep track of problematic
> media areas and rewrite or reallocate affected blocks.  That includes
> stuff like rewriting sectors that are susceptible to bleed due to
> being adjacent to write hot spots.

Good to know. Could yo tell me if this is actually true for normal
state-of-the art SATA disks, or only true for more expensive disks?
Do you have a good reference for it.

best regards
keld
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