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.