VM zvols, sometimes have refreservation, sometimes don't; how to reduce?

General Discussion
,
1

In my TrueNAS Core, I have a bunch of VMs, backed by a zvol naturally.

Running:

zfs list -r -o name,used,available,refreservation tank

I see two kinds, some with no REFRESERV and some with, ex:

NAME          USED  AVAIL  REFRESERV
olympus/VM1  1.75T  1.54T       609G
olympus/VM2  1.99T   973G       none

First, I’m not sure how that even happened. What part of the TrueNAS UI would have set that?

Anyway, it is how it is. VM1 above has a large REFRESERV, and yet AVAIL is very large. I suspect this is wasting space. How can I recover this space?

Thanks.