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.