Thanks for adding some info, Claudius.
In that case please open a ticket with support - they need to figure out what's happening. What OS rev?
Regarding dedupe: The idea is that you run it when the system is not busy doing other things. So, I'd rather you run it before you start snapmirror sessions, or do any serious I/O, and, for efficiency reasons, before doing snaps.
However, even dedupe should back off if you want to make a request from the system.
Maybe you can try wafltop...
priv set -q diag
options stats.wafltop.config volume,process,message
wafltop start
(wait 5 min)
wafltop stop
options stats.wafltop.config off
You'll get an idea of what's happening in the system, and at the very least, if you can't understand what it's saying, the support personnel will get a head start.
Also be prepared to give support a perfstat pls.
D