Hi Ludo, > Do you see a modprobe invocation for “fs-btrfs” or “block-major-xxx” > before? I can't get the logfile to show any modprobe log message in this case (which is "make TESTS=btrfs-root-os check-system"). I had that problem before and I think it's because guix is using some special /tmp/repl pipe for qemu here[1] and so the message vanishes somewhere where I can't see it (that is, the guest does everything correctly but the host loses it somewhere). The usual trick I use in this case which is "steal the cmdline and the files from /proc/ and invoke it myself" doesn't work either because of the pipe... hrmmmm... I tried both /dev/kmsg and just not closing current-output-port etc. I've double-checked that I've applied all v11 patches, including the resolve-alias one (which I think is necessary anyway). Help? Log file attached. [1] When it uses the VM to create the root fs.