Open ZFS uses the Registry for default tunable values which is read in on startup.
The Registry key hostid is generally handled by OpenZFS internally, ie, generated randomly if it is not set the first time Open ZFS is loaded.
You can change hostid if that is so desired. For example, if you share the pool between multiple Operating Systems on the same Hardware.
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.