Windows Registry
From OpenZFS on OS X
Registry
Open ZFS uses the Registry for default tunable values which is read in on startup.
\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\ZFSin
The Registry key hostid is generally done automatically, ie, generated randomly if not set the first time Open ZFS is loaded. You can change if that is so desired. For example, if you share the pool between multiple Operating Systems on the same Hardware.
Arc Max
To set the ARC maximum memory usage to 2GB, create an entry;
"zfs_arc_max" REG_QWORD "0x80000000"
and reboot. If you wish to change it on a running system without rebooting, see the kstat example.