I've read that BTRFS requires at least 4 disks (https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices). Indeed, Linux mdadm raid10 requires only 2 disks (http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10). Is there something like "raid10,f2" provided by Linux mdadm in BTRFS? I'd like to have the checksumming capability built-in in the raid array, if possible.
Well four is the smallest number of devices for which RAID10 makes sense really.
Because it involves mirroring you need an even number of disks, and if you only have two disks then what you have is just a mirror, which is a RAID1 array.
Yes they could allow a two disk RAID10 array, but it would be exactly equivalent to a two disk RAID1 array so I imagine they have simply chosen not to allow that option.