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]

Hello Antonio,

thank you for your response.

> It is already planned to implement such a feature. I have
> been unable to work on this until now, but I expect it to
> be done in a few weeks. Please, read this message[1]
> and write back any comments you have.
> [1]https://lists.gnu.org/archive/html/bug-ddrescue/2011-04/msg00004.html

Ah, I did not browse through the archive that far into the past...

I read through that message now and would have the following comments:


1) Minimum reading speed / jump threshold

1a) It would be nice if the default value would be something like "Skip 
x Byte if current speed < (average speed / 10)". This should work nicely 
on any media regardless of its speed. Also initially the average speed 
is 0, so in the default scenario (no offset specified) it will not jump 
right over the MBR, etc.

1b) It would be nice if the desired min speed could be set in kByte/s on 
the command line.

1c) Please allow the entire feature to be disabled by specifying a min 
speed of 0. (I guess this shouldn't be much work to implement unless you 
get negative current read rates ;-) )


2) Jump size
It would be nice if it was possible to specify the number of sectors (or 
kByte/MByte) to be skipped if the current reading speed is below the 
specified minimum speed (or below 10% average speed if not specified).

I had exchanged a few mails on this topic with Kim Pedersen (who 
suggested this to be made a parameter), wondering what a good default 
value would be.

Unfortunately my bad drive has relocated all bad sectors by now so I 
can't do any useful testing :-(
In the first run the "very slow" areas managed to bring the drive down 
to ~6-10MByte/s average over the entire 2TB.
If I run it again now I get ~35-40MByte/s average (and no read errors 
anymore, which brings me to the next topic)


3) Disable relocation of bad sectors?
This is probably a bit out of the scope, but as described above I have 
noticed that my drive relocated a lot of weak sectors internally and 
also the few unreadable ones.

I understand that this behavior will be desired in normal operation, but 
e.g. when the drive has a temperature issue it might be quite bad if a 
(temporarily, e.g. due to temperature) 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?


4) Disable read-ahead
(This is another idea raised by Kim.)

By default the Kernel seems to read ahead into the drive while accessing 
the data requested by ddrescue - which according to Kim does not make 
much difference regarding performance on good areas on the disk.
But if the reads become slow, the read ahead by the kernel would cause 
the drive to read further into the slow area than requested by ddrescue.

So maybe it would be a good idea to disable read-ahead for the first 
pass - or maybe to just mention this in the documentation?


Kind Regards
Felix Ehlermann