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

[Burkart Lingner]

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