For software volume control, I put jackminimix between zita-a2j and zita-j2a or between zita-a2j and netone.

I didn't know why I got xruns while I compiled kernel or gentoo packages.

Today, I used stress-ng to put various loads on my computer.

"nice -n 19 stress-ng --iomix 8 --ionice-class idle -t 5m" caused multiple xruns.

"nice -n 19 stress-ng --cpu 8" didn't cause xruns.

I tried giving real-time ionice priorities to netone backend, zita-ajbridge, and jackminimix.

Eventually, the setup that seems to withstand "nice -n 19 stress-ng --iomix 8 --ionice-class idle --cpu 8 -t 5m" was

ionice -c 1 -n 3 sudo -u user jackd -P 90 -d netone ("-p 256" inherited from jack_netsource) -n 3
ionice -c 1 -n 0 sudo -u user stdbuf -o 0 zita-a2j -j from_alsa -d spdif_in -p 128 -n 2 -r 48000
ionice -c 1 -n 0 sudo -u user stdbuf -o 0 zita-j2a -j headphones -d xfihd_hdp -p 128 -n 2 -r 48000
ionice -c 1 -n 6 sudo -u user stdbuf -o 0 jackminimix -c 1 -p 20255 -n hdp-vol
ionice -c 1 -n 6 sudo -u user stdbuf -o 0 jackminimix -c 1 -p 20256 -n spk-vol

I will have to test more to be sure that this setup really withstands writing to ZFS which amplifies writes many times.
I guess that writing to ZFS interferes with reading from and writing to USB soundcard or ethernet adaptor. I thought ionice only affected file system operations.

Perhaps, does ionice affect xruns because I log stdout and stderr of jackd, zita-ajbridge, and jackminimix into files on ZFS without buffering? "stdbuf -o 0" removes buffering.