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 Felix,

My interpretation of what happens with read-ahead is the device will be 
told to fetch say, blocks 100-109, if 103-106 has errors, you'll need to 
wait for the retry-timeout to take place on all of those blocks. With 
direct access (-d) you bypass the read-ahead and ddrescue will complete 
it's first pass a lot quicker as it will skip ahead on encountering the 
first errors at block 103.

Anyone else, please correct me if I am wrong.  Performance wise I've 
never seen much difference between using -d or not on a good disk, so I 
always use it. YMMV


Kim


On 2011-09-26 16:43, Felix Ehlermann wrote:
> 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
>
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