启用softwhen似乎是有利的，没有不利的：balance将跳过已经在正确位置的块。
man btrfs-balance说：
soft Takes no parameters. Only has meaning when converting between profiles. When doing convert from one profile to another and soft mode is on, chunks that already have the target profile are left untouched. This is useful e.g. when half of the filesystem was converted earlier but got cancelled. The soft mode switch is (like every other filter) per-type. For example, this means that we can convert metadata chunks the "hard" way while converting data chunks selectively with soft switch.
是否存在任何情况，人们会不是希望通过soft？