Hi Marco,
mdrecoveryd is a kernel process.
It shouldn't be using any resources if you don't have RAID configured, but it will still run because RAID is configured in most kernels, incl. the RH kernel SME uses, by default.
Nice value you report is strange, it should be between 20 and -20.
The higher boundary seems to have "flipped" in your case.
That however isn't all that unusual it seems.
It has been reported for RH 7.2, 7.3 kernels.
A little googling will bring examples like
this.
I am afraid I can't help you in what went wrong,
but the only way you can get rid of mdrecoveryd is recompling a new kernel

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Regards
Reinhold