And also why RAID1 and RAID4/5/6 need the data bounced. I wish VFS
would prevent data writing given a device queue flag that requests
it. So all these devices and modes could just flag the VFS/filesystems
that: "please don't allow concurrent writes, otherwise I need to copy data"
From what Chris Mason has said before, all the mechanics are there, and it's
what btrfs is doing. Though I don't know how myself?