So I'm trying to figure out the advantage of including RAID 1 inside btrfs instead of just running it over a conventional (fs-agnostic) RAID subsystem. I was originally really intrigued by the idea of integrating RAID into the file system since it seemed like you could do more that way, or at least do things more efficiently. For example, when adding or replacing a mirror you'd only have to copy those parts of the disk that actually contain data. That promised better performance. But if those actually-used blocks are copied in small pieces and in random order so the operation is far slower than the logical equivalent of "dd if=disk1 of=disk2', then what's left? Even the ability to use drives of different sizes isn't unique to btrfs. You can use LVM to concatenate smaller volumes into larger logical ones. Phil
