Default [Q] BTRFS on xperia p?

I tried to use btrfs for /data on my xperia p(with btrfs support compiled in kernel), but the CM11 ROM doesn't seem to be able to read or write the partition.
There are some files generated by CM11 in /data after I force reboot into recovery. But I waited for 20 min and the CM11 boot animation is stiill there.
I only modified the / file in ramdisk to change the filesystem from ext4 to btrfs. Is there anything wrong with it?
What's wrong with my operation and what should I do?