ndbkit+mock

  https://www.redhat.com/archives/libguestfs/2020-May/msg00053.html
  https://www.redhat.com/archives/libguestfs/2020-May/msg00074.html

Easy set up is:

  # rm -f /tmp/sock
  # nbdkit -U /tmp/sock memory 100G
  # nbd-client -b 512 -unix /tmp/sock /dev/nbd0 -connections 4
  # mkfs.xfs /dev/nbd0
  # mount /dev/nbd0 /var/lib/mock

or using http://libguestfs.org/nbdkit-tmpdisk-plugin.1.html:

  # rm -f /tmp/sock
  # nbdkit -U /tmp/sock tmpdisk size=100G
  # nbd-client -b 512 -unix /tmp/sock /dev/nbd0 -connections 1
  # mount /dev/nbd0 /var/lib/mock

or (requires bleeding edge nbdkit):

  # rm -f /tmp/sock
  # lvcreate -L 100G -n tmp /dev/fedora
  # nbdkit -U /tmp/socket --filter=fua fuamode=discard file /dev/fedora/tmp
  # nbd-client -b 512 -unix /tmp/sock /dev/nbd0 -connections 4
  # mkfs.xfs /dev/nbd0
  # mount /dev/nbd0 /var/lib/mock