I do a pool called backup with two raidz vdevs - 7x3T in one raidz vdev, and 9x4T in one raidz vdev. Data is striped on each of the two raidz and I lose one 3T and one 4T's worth of capacity. This allows me to lose any one drive at a given time without data loss, and up to two drives (as long as they come out of separate vdevs).

As far as I could work out from the btrfs documentation, this isn't currently possible? Plus, RAID5/6 in btrfs is still of questionable stability?

Thanks for clarification!

Indeed, btrfs doesn't allow such configuration. However, it allows using disks of unequal sizes within single filesystem - like, 3+3+4T in RAID1 mode gives you 5T of usable space (imagine 4T disk being split in half, and each half duplicated to a different 3T disk; remaining space of 3T disks duplicated to each other). But as I understand, it's possible to achieve the same with manual partitioning and vdev allocation on ZFS, too.

RAID5/6 is indeed a danger zone - in another thread I've already mentioned a nice write-up of "guidelines for users running btrfs raid5 arrays to survive single-disk failures without losing all the data" by Zygo Blaxell: https://lore.kernel.org/linux-btrfs/20200627032414.GX10769@h...