However you get no striping, and data is only read from one drive, so performance is limited to that of one drive for reads and writes. Plus with mismatched drives, smaller drives go unused unless you write enough data.
You're going to have to support that claim a bit better. The core idea of RAID 1 is mirroring data, which BTRFS RAID 1 mode definitely does. Striping is not an essential part of RAID 1 (hence RAID 10), and reading data from two disks in parallel is an optional performance optimization that is not performed by all RAID 1 implementations (but could be implemented for BTRFS RAID 1: https://stackoverflow.com/questions/55408256/btrfs-raid-1-wh... ).
> Plus with mismatched drives, smaller drives go unused unless you write enough data.
Yes, the allocation is suboptimal from a performance perspective, as I've already said. But it is simple and straightforward and reasonably good at avoiding putting you into a situation where manually issuing a rebalance command is necessary. If you do need better performance, there's a RAID 10 mode. But since my NAS is currently a motley pile of SSDs, I don't need to to anything extra to have decent performance.
It'd be way easier to talk about if it had a unique name, and you could say "It's like RAID1".
Despite all that I do like the mode, and use it in a few places.
You have raid1c<drive count> mode in BTRFS which does this.