We have had an issue with our FreeNAS server kernel panicing when doing multiple large file transfers. We are running 64-bit machine with 24TB of available space.
First you will need to set the FreeNAS filesystem to writeable so you can make the changes to system files.
mount -o rw /dev/ufs/FreeNASs1a /
Now that your filesystem is writeable you will need to modify the /boot/loader.conf file to include the following.
vm.kmem_size_max="1024M" vm.kmem_size="1024M" vfs.zfs.arc_max="100M"
This seems to have fixed the issue with kernel panics for now.