How to format spare disk space to btrfs filesystem? I can't see btrfs in format dialog neither in gparted, nor in Disk Utility. Only Fat,ext2,ext3,ext4 etc.
It may be because btrfs is not really stable yet. So I guess the GUI tools do not support it yet, and you will have to use command line tools if you want to use it.
You might need to install the package btrfs-tools . This should enable integration with gparted and etc.