[Bug-ddrescue] Slowing down of ddrescue over time

[Richard Bertrand]

Firstly, thank you for this fine tool.
I use it on almost all recoveries, as I never know whether the drive is 
at fault or not when I get a non-working computer which I am asked to 
revive.

There is one peculiarity I have noticed, however: ddrescue is starting 
fast and subsequently slowing down during the copy process.
I have searched the web for an explanation, and I found only a few other 
people that noticed the same behaviour, but no answers as to what 
process or condition is causing this.

My set-up is simple: I usually have a laptop or desktop that I start-up 
with Parted Magic 5.x or so. It has ddrescue version 1.11. I then hook 
up a usb disk on an usb v2.0 port on the machine and start ddrescue with 
a log file. The usb disk is ntfs formatted (because I need it also to be 
accessible in Windows), so all data is going through the mount.ntfs-3g 
driver.

Even so, most source disks are also ntfs-formatted as these 
to-be-revived computers run Windows....

It starts copying at good speed (about 30000KB/s) and then immediately 
starts to loose speed, even on perfectly good source disks. At the end, 
with a 250GB or larger (max 500GB) source disk, it is barely doing 1000KB/s.

I have speed tested my usb disk and it seems capable of sustaining 
30MB/s (the usb2.0 interface is the limiting factor for this sata disk) 
so that should not be the problem. I noticed that, on a two-core cpu'ed 
machine, the CPU load is about 50%, where the load is almost al used for 
the mount.ntfs-3g and ddrescue. I think (but I am not sure about that) 
that the load at the end seems to stem more from the mount.ntfs-3g 
driver then at the beginning of the rescue.

When I stop the rescue process and start it over again (with the log 
file so that the process starts from the point it previously was 
interrupted), ddrescue is again, for a much shorter time period, copying 
with 30MB/s. It then falls very rapidly to the same speed it was going 
when I interrupted it.

Is there something I can do to keep it rescuing data at maximum speed? 
What is getting in the way of ddrescue that it is slowing down so much?

Thanks and regards,
Richard Bertrand