One interesting titbit I've only recently found out is that btrfs can't really serve reads from different drives in RAID1, it picks a drive based on the process

One interesting titbit I've only recently found out is that btrfs can't really serve reads from different drives in RAID1, it picks a drive based on the process id. ZFS does something smarter here, it keeps track of the queue length for each drive in a mirror, and picks the one with the lowest number of pending requests.