BTRFS RAID1 has numerous advantages compared to regular RAID1. It does not mirrors whole disks, but chunks of the disk, usually 1GB chunks. This has numerous implications:
- drive size can be mixed without usable space loss (kind of, with some combinations you end up losing a bit)
- you don't need an even number of drives
- you don't need a raid0 atop several raid1 to use e.g. 6 drives, it's all one big raid
- a drive can be swapped for a bigger or smaller drive without issue, meaning spare drives don't need to be identical
etc.