Re: Distributed spares

["David Lethe" <david@santools.com>]

-----Original Message-----

From:  Keld Jørn Simonsen <keld@dkuug.dk>
Subj:  Re: Distributed spares
Date:  Tue Oct 14, 2008 8:06 am
Size:  1K
To:  "Martin K. Petersen" <martin.petersen@oracle.com>
cc:  "Billy Crook" <billycrook@gmail.com>; "Justin Piszcz" <jpiszcz@lucidpixels.com>; "Bill Davidsen" <davidsen@tmr.com>; "Neil Brown" <neilb@suse.de>; "Linux RAID" <linux-raid@vger.kernel.org>

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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read the manual for any disk drive... They go into error detection,  correction recovery algorithms and capability in  great detail.  
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