ZFS Tuning Guide
http://wiki.freebsd.org/ZFSTuningGuide
If you need to extend them beyond 512M, you need to recompile your kernel with increased KVA_PAGES option, e.g. add the following line to your kernel configuration file to increase available space for vm.kmem_size beyond 1 GB: options KVA_PAGES=512
There is one example (CySchubert) of ZFS running nicely on a laptop with 768 Megs of physical RAM with the following settings in /boot/loader.conf:
vm.kmem_size="330M" vm.kmem_size_max="330M" vfs.zfs.arc_max="40M" vfs.zfs.vdev.cache.size="5M"
Kernel memory should be monitored while tuning to ensure a comfortable amount of free kernel address space. The following script will summarize kernel memory utilization and assist in tuning arc_max and VDEV cache size.
#!/bin/sh -
TEXT=`kldstat | awk 'BEGIN {print "16i 0";} NR>1 {print toupper($4) "+"} END {print "p"}' | dc`
DATA=`vmstat -m | sed -Ee '1s/.*/0/;s/.* ([0-9]+)K.*/\1+/;$s/$/1024*p/' | dc`
TOTAL=$((DATA + TEXT))
echo TEXT=$TEXT, `echo $TEXT | awk '{print $1/1048576 " MB"}'`
echo DATA=$DATA, `echo $DATA | awk '{print $1/1048576 " MB"}'`
echo TOTAL=$TOTAL, `echo $TOTAL | awk '{print $1/1048576 " MB"}'`