Why xfs can not mount use barrier option?

[ch huang]

i use lvm on raid 1,/dev/CHVG01/conxfs is a logical volume,if i  mount with
barrier,the /proc/mounts still show nobarrier option ,why?

#  mount -t xfs -o barrier,attr2,noikeep,inode64 /dev/CHVG01/conxfs
 /opt/contest/
# cat /proc/mounts
/dev/CHVG01/conxfs /opt/contest xfs rw,attr2,nobarrier,inode64,noquota 0 0
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