I'm setting up a bhyve wherein:

host #  truncate -s 1T vol.file
host #  du -ah vol.file
200K    vol.file

host #  /usr/sbin/bhyve ... -s 4,ahci-hd,vol.file ...

Then inside the bhyve I create a zpool (ada0 = vol.file):

bhyve #  zpool create -O devices=off -O atime=off -O compression=on -m
/mnt/data1 data1 ada0

And I put a bunch of stuff in the zpool ... and the vol.file grows in size:

host #  du -ah vol.file
100G    vol.file

Then I remove the files from the zpool and the zpool usage returns to 0 but
of course the vol.file size does not shrink, the data is still there (but
not referenced?)

Normally I'd just write zeros to a file inside the zpool until the pool
fills up, then maybe cp --sparse vol.file for good measure, but with
compression on in the zpool the zeroing doesn't really fill up space or
seem to overwrite anything. In my testing the zero file grew larger than
100G with no change to vol.file  I did not let it run forever, however.

Any other ideas how to scrub off or clear out deleted data from a zpool
and/or this kind of file-backed device?
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