ZFS on Linux: How to find the arc stats (was arcstat.py)

This has now changed; run the following to find the adaptive read cache stats (ARC):  
cat /proc/spl/kstat/zfs/arcstats
  You can gleam some really useful information out of how your RAM is being utilised and what your required ARC size might be from the results – this may be a topic for a future post, however!