It was suggested to me by Ian Collins, that doing zfs sends and
receives, can render a filesystem "busy".
if there isnt a process visible doing this via ps, I'm wondering how
one might check if a zfs filesystem or snapshot is rendered "busy" in
this way, interfering with an unmount or destroy?
I'm also wondering if this sort of thing can mean interference between
some combination of multiple send/receives at the same time, on the
zfs-discuss mailing list