Yup, I used to use LVM, but the two big NAS filesystems have a ton of nice features and they expect to control the disk management. I looked into BTRFS and ZFS, and since BTRFS is native to Linux (some of my SW doesn't support BSD) and I don't need anything other than RAID mirror, that's what I picked.

I used LVM at work for simple RAID 0 systems where long term uptime was crucial and hardware swaps wouldn't likely happen (these were treated like IOT devices), and snapshots weren't important. It works well. But if you want extra features (file-level snapshots, compression, volume quotas, etc), BTRFS and ZFS make that way easier.

Actually, the lower level may likely be less efficient, due to being oblivious about the nature of the data.

For example, a traditional RAID1 mirror on creation immediately starts a rebuild across all the potential data capacity of the storage, without a single byte of actual data written. So you spend an entire drive wipe making "don't care" bytes redundant.

Similarly, for snapshotting, it can only track dirty blocks. So you replace uninitialized data that means nothing with actual data, the snapshot layer is compelled to back up that unitiialized data, because it has no idea whether the blocks replaced were uninialized junk or real stuff.

There's some mechanisms in theory and in practice to convey a bit of context to the block layer, but broadly speaking by virtue of being a mostly oblivious block level, you have to resort to the most naive and often inefficient approaches.

That said, block capacity is cheap, and doing things at the block level can be done in a 'dumb' way, which may be easier for an implementation to get right, versus a more clever approach with a bigger surface for mistakes.