Routine backups of the VM's and raid disk for the hypervisor running them. If the box hosting the backups went screwy there's a problem but with something like 20TB of space used copies off-box are a bit cumbersome. To that end I just manually copy the irreplaceable stuff to a separate external storage and wish the movies and stuff good luck.
It ends up with a situation though where I'd have to lose both the disks on the hypervisor and if that happened several disks on the NAS (12 disks in a ZFS pool with each vdev being a mirror pair) or for the whole pool to get screwed up to lose the VMs fully. Depending on the day I might lose up to a week of VM state though since they only do a full copy once a week.