Improving ZFS Efficiency

Create new filesystems for tmp and var

zfs create tank/SYSTEM/tmp -o mountpoint=legacy
zfs create tank/SYSTEM/var -o mountpoint=legacy

Mount the new filesystems.

mkdir -p /mnt/{tmp,var}
mount -t zfs tank/SYSTEM/tmp /mnt/tmp
mount -t zfs tank/SYSTEM/var /mnt/var

Copy the filesystems

rsync -ar /tmp/ /mnt/tmp/
rsync -ar /var/ /mnt/var/

Update /etc/fstab

# <fs>   <mountpoint>  <type>  <opts>        <dump/pass>
tank/SYSTEM/tmp /tmp    zfs     nodev,relatime  0 0
tank/SYSTEM/var /var    zfs     nodev,relatime  0 0

You'll need to reboot for changes to take effect.