[Bug-ddrescue] ddrescue slows down by-and-by and will never finish

[GT]

Hey folks,

I have the following question and we could not find any hint about that 
problem yet:

We try to rescue a 250 gig hard disk which seems to contain only few 
errors (another programm found some at about 2% and 50%) and started it 
with the command

sudo ddrescue -v /dev/sda1 /media/extern/backup/platte22.dd 
/media/extern/backup/logfile

The hard disc first was left in my PC and the rescue hard disc is an 
extern USB TrekStor 1TB. We used a UBUNTU boot CD, because the hard disc 
did not work within my Windows system anymore. Now the disc runs in a 
Linux system also with the extern USB, but this does not make any 
difference with the speed.

It started fast with about 30000 MB/s and then slowed down by-and-by to 
100kB/s by now at 100 gigs.
Important fact: ddrescue holds the speed for some time, then falls down 
a step, holds the speed again, falls down a step again and so on. So it 
slows down step-by-step and never speeds up again. This is why the first 
100 gigs are rescued in an aceptable time, but with the current speed it 
would take weeks to finish (and it still slows down on and on, step by 
step - so it will never finish I think).

It really feels like you take a piece of paper and cut it in two pieces. 
Then again. And again. The rest becomes smaller and smaller, but you 
will never finish. There will be always a rest of the paper that you can 
cut into two pieces.


So what could be the problem and what a possible solution for it?

My opinion is, that it is because of the big file size. But that would 
be a problem for everybody, so why did no one report this? But if it 
would be a problem because of errors it would have found more than 1 
error until now or at least speeded up again in between, I think? If you 
use Ctrl-C and restart it, it only ties up to the last rate, also if you 
let it cool down in between. So overheating can also not be the problem.
If you restart from the beginning it will also start as described above 
at 30000 MB/s and do exactly the same as I wrote and does not find more 
errors than before. So the disc is not going to die soon and slows down 
because of that, I think.

So is there any chance to make ddrescue speed up again or work with an 
constant rate?

This is the actual status:

Initial status (read from logfile)
rescued:   103433 MB,  errsize:    102 kB,  errors:       1
Current status
rescued:   103437 MB,  errsize:    102 kB,  current rate:    65536 B/s 
  ipos:      103437 MB,  errors:       1,     average rate:     103 
kB/s    opos:      103437 MB



Best regards from Germany,
Gernot