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

[Felix Ehlermann]

Hi Kim,

thank you for your suggestion.

I have not tried the -d switch yet, but I assume that this setting would 
affect transfers over the entire disk, not just specific sectors.
At the beginning of the drive I get a continuous "current rate" of ~105 
MByte/s - not that slow for a magnetic disk IMO.
So I don't think that there's a general perfomance issue because of the 
read ahead being enabled.

But I do understand that the read ahead might slow down transfers on 
damaged areas - yet the HD did not return IO errors on those slow sectors.
It was probably able to relocate them successfully - I have started a 
second run over the HD which is right now at an average speed of ~37 
MByte/s. In the previous run I got ~10 MByte/s average speed.
(I started with an empty logfile in order to re-read the entire disk 
into a second copy)

Actually this rises another interesting question to me:
Is there a way to read data from the drive without having the drive 
relocate weak / bad sectors upon reading?
I would be worried that a weak sektor was read with invalid data but 
correct (drive internal) checksum at the first attempt - in this case 
relocation would prevent another attempt at this sector as the (wrong) 
data read from it will be stored in the spare sector the drives firmware 
uses as a replacement.

Yours
Felix

On 26.09.2011 19:16, Kim Pedersen wrote:
> Hi Felix,
>
> Glad that you are happy with ddrescue - it is very good and very 
> useful indeed.
>
> What command are you using? Have you tried to use -d to disable 
> read-ahead?
>
> Many many years ago I came across the same issue as you, and find 
> recoveries go better (faster) when you disable the OS tendency to read 
> ahead (Sometimes many MBs)
>
>
> Kim