Disappointed this doesn't talk about nbdkit, the easiest way to spin up block devices for testing:

https://libguestfs.org/nbdkit-loop.1.html

eg. Make a 7 exabyte disk:

  $ sudo modprobe nbd
  $ nbdkit -U /tmp/sock memory 7E
  $ sudo nbd-client -unix /tmp/sock /dev/nbd0
  $ sudo mkfs.btrfs /dev/nbd0
  $ sudo mount /dev/nbd0 /tmp/mnt
  $ df -h /tmp/mnt
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/nbd0       7.0E  3.4M  7.0E   1% /tmp/mnt

I even did a talk about it:

https://archive.fosdem.org/2019/schedule/event/nbdkit/