Proxmox was installed on a single SSD using the ext4 FS (default), what should be the most adequate FS for the OPNsense VM, UFS or XFS? Thank you
First off: You lose much by not installing Proxmox in ZFS in the first place. By using that you can always rollback your VMs and are sure that no filesystem corruptions stay undetected and use thin provisioning and compression, too.
While you could use anything as filesystem in the VMs, I would also use ZFS on OpnSense, because you can use the new snapshot feature within the VM itself. Also, UFS can get corrupted sometimes on unclean shutdowns. That being said, you could still rollback the whole VM under Proxmox.
Agreed, ZFS - when I went from physical to Proxmox I took the opportunity to change from UFS to ZFS
I would go with UFS, simply because I assume that virtualized ZFS is not a great idea and since you virtualize OPNsense, you can use Snapshots/Backups in Proxmox.