Arch Linux

[aarcane@kropstnega media]$ sudo mkfs.xfs -L xfs -d unwritten=0 -i attr=2,maxpct=5 -l version=2 -r rtdev=/dev/mapper/spare-xfs--realtime /dev/spare/xfs -f
meta-data=/dev/spare/xfs isize=256 agcount=8, agsize=6400 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=51200, imaxpct=5
= sunit=0 swidth=0 blks, unwritten=0
naming =version 2 bsize=4096
log =internal log bsize=4096 blocks=1200, version=2
= sectsz=512 sunit=0 blks
realtime =/dev/mapper/spare-xfs--realtime extsz=65536 blocks=6144, rtextents=384
[aarcane@kropstnega media]$ grep xfs /etc/fstab
/dev/spare/xfs /media/xfs xfs defaults 0 0
[aarcane@kropstnega media]$ sudo mount xfs
mount: Function not implemented
[aarcane@kropstnega media]$ dmesg | tail -n 3
eth0: no IPv6 routers present
Filesystem "dm-2": Disabling barriers, not supported by the underlying device
XFS: RT mount failed
[aarcane@kropstnega media]$

the above commands should have created a filesystem of type XFS on the device /dev/mapper/spare-xfs and a region for XFS filesystem realtime data in /dev/mapper/spare-xfs--realtime. however, it either did not create the realtime volume, or the kernel refuses to mount it for some reason. I'm not sure which. the end result is a documented feature is severely broken in it's current state.