Re: [Bug-ddrescue] Feature Request: Introduce parameter to skip "slow" (still readable) sectors during first pass, if reading speed is below x kByte/s

[Kim Pedersen]

Hi Burkart,

You are correct - you can't disable reallocation of bad sectors, it's 
controlled by the harddrive.

If a sector is unreadable *now*, it is marked pending unreadable (It may 
be readable later, as temperature changes make the drive platters 
expand/contract).

If you write to a pending sector, it will be reallocated, so as long as 
you don't write (as for example in a format) to your disk.


Have a look at the smartmontools BadBlock Howto here for more 
information on the process:
 http://smartmontools.sourceforge.net/badblockhowto.html


Regards,

Kim







On 2011-09-28 04:29, Burkart Lingner wrote:
> Hi Felix!
>
> > 3) Disable relocation of bad sectors?
> > [...]
> >
> > [...] it might be quite bad if a
> > [temporarily] unreadable sector got replaced by
> > a space and therefore is not accessible anymore.
> > Maybe there is a way to tell the drive not to relocate sectors on error?
>
> I have no first-hand information on how relocation works but my 
> understanding of it is this: If a sector can't be read successfully 
> due to ECC errors the drive returns an error and saves that 
> information in the SMART logs. If a sector is ECC-correctable but only 
> barely so or maybe if the sector couldn't be read previously and was 
> internally marked as such then the drive's firmware will relocate the 
> restored (i.e. good) data to a spare sector and internally 
> decommission the original sector. A relocation also takes place if a 
> known bad sector is written to.
>
> In essence, relocations only happen if the drive firmware is sure to 
> know what data the sector should contain. If a sector remains 
> unreadable for all eternity and isn't written to it is also never 
> relocated.
>
> However, sectors can also be relocated withouth user interaction by a 
> hard disk's offline data collection. When a hard drive is idle for a 
> certain period of time it automatically tries to read all sectors and 
> relocates them if the data can be read successfully but if the sector 
> is known to be unreliable. As sectors are only relocated when read 
> successfully I don't see anything wrong with this feature in general, 
> but if you want to you can turn it off using i.e. smartmontools. If 
> you do that, don't write to a damaged disk, and don't successfully(!) 
> read bad sectors then no relocations should happen.
>
> So if my information is correct and there are no implementation bugs 
> then you should be happy for every relocated sector as it now holds 
> the data the bad sector was supposed to contain.
>
> Bye, Burkart
>
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