> Easiest way to create sparse eg 20 GB assuming test.img doesn't exist
> already
No no no. Easiest way to do what you want to do:
mdconfig -a -t malloc -s 3t -u 0
mdconfig -a -t malloc -s 3t -u 1
Just make sure to offline and delete mds ASAP, unless you have 6TB of
RAM waiting to be filled ;) - note that with RAIDZ2 you have no
redundancy with two fake disks gone, and if going with RAIDZ1 this
won't work at all. I can't figure out a safe way (data redundancy all
the way) of doing things with only 2 free disks and 3.5TB data - third
disk would make things easier, fourth would make them trivial; note
that temporary disks 3 and 4 don't have to be 2TB, 1.5TB will do.
I've done this dozen of times, had no problems, no gray hair, and not
a bit of data lost ;)