[Bug-ddrescue] Slowdown after ~1 hour

[Hendrik Müller]

Hello,
I'am currently in the process of a poor man's LSI raid data recovery.
After a power supply failure I had to replace the head stack assembly of
the drive (Western Digital Red 3TB).
The drive is now working again but I have the problem that ddrescue 1.21 is
copying with ~300kB/s for the first hours and then suddenly drops to
~1000B/s.
Speed stay at this rate for ever, I had running this for over a week.
However when interrupting ddrescue and start again immediately speed is
again at ~300kB/s for ~1 hours and then drops again.
It is not always exactly an hour, sometimes it already happens after 30
minutes, sometimes after 3 hours.
Anyway, the pattern is that after interrupting and starting again there is
"full speed" again (drive is powered on the while time, no interrupt).

First I tough that it might be because of my setup (Raspberry Pi 3, latest
Raspbian + Firmware, two ASM1053 USB to SATA adapter) but the same happens
on a regular x86 machine running SystemRescueCd and directly connecting the
drives via SATA.
I compiled ddrescue 1.21 in both cases by myself.

Any idea what could be the problem here?