Taking ZFS Arc cache into account
In Linux systems using the ZFS filesystem (including latest releases of Ubuntu, if chosen during install) the free memory computed by most tools, including free and this plugin, is wrong, because the ZFS "Arc" cache is not included in the Cached item of /proc/meminfo as it should be.
There have been many discussions and bug reports on this topic for years now, that are getting nowhere. In the meantime, I patched the plugin so that whenever ZFS stats are available in /proc, it will add the ZFS Arc cache to the MFree value, alongside the more traditional MCached and MBuffers values.
Please find attached a Git patch.