Major On Zabbix 2.0.0 I found that I cannot check an XFS partition of a certain mounted file system. See the below example for /var/lib/hadoop/1. The root 
file system is XFS, too, but works properly
root@zabbix:/root>zabbix_get -s foo.example.org -k vfs.fs.discovery
{
"data":[
{
"{#FSNAME}":"\/",
"{#FSTYPE}":"rootfs"},
{
"{#FSNAME}":"\/sys",
"{#FSTYPE}":"sysfs"},
{
"{#FSNAME}":"\/proc",
"{#FSTYPE}":"proc"},
{
"{#FSNAME}":"\/dev",
"{#FSTYPE}":"devtmpfs"},
{
"{#FSNAME}":"\/dev\/pts",
"{#FSTYPE}":"devpts"},
{
"{#FSNAME}":"\/",
"{#FSTYPE}":"xfs"},
{
"{#FSNAME}":"\/lib\/init\/rw",
"{#FSTYPE}":"tmpfs"},
{
"{#FSNAME}":"\/dev\/shm",
"{#FSTYPE}":"tmpfs"},
{
"{#FSNAME}":"\/boot",
"{#FSTYPE}":"ext2"},
{
"{#FSNAME}":"\/var\/lib\/hadoop\/1",
"{#FSTYPE}":"xfs"},
{
"{#FSNAME}":"\/mnt\/test",
"{#FSTYPE}":"nfs4"},
{
"{#FSNAME}":"\/var\/lib\/hadoop\/2",
"{#FSTYPE}":"nfs4"},
{
"{#FSNAME}":"\/var\/lib\/nfs\/rpc_pipefs",
"{#FSTYPE}":"rpc_pipefs"}]}
root@zabbix:/root>zabbix_get -s foo.example.org -k "vfs.fs.size[/var/lib/hadoop/1,pfree]"
ZBX_NOTSUPPORTED
root@zabbix:/root>zabbix_get -s foo.example.org -k "vfs.fs.size[/,pfree]"
89.987682
root@zabbix:/root>zabbix_get -s foo.example.org -k "vfs.fs.size[/boot,pfree]"
92.789128
The debug log on the Zabbix agent didn't tell me much - just that the item was unsupported.
I just checked and the problem is reproducible with Zabbix agent version 1.8.2, too.