I'm looking at intros to #MergerFS:
that's quite interesting stuff, but a single file can not grow to span multiple members.
Example:
member1 1GB
member2 2GB
MergerFS 3GB
a 2.5GB file will neither fit, nor can it be grown beyond the capacity of the member it's 1st stored on!
The "moveonenospc" option only helps up to the length of the largest free capacity on a single member.
I'm not switching from #ZFS, any time soon. 😉