> The dm layer gives you cow/snapshots for any filesystem you want already and has for more than a decade. Some implementations actually use it for clever trickery like updates, even.

O_o

Apparently I've been living under a rock, can you please show us a link about this? I was just recently (casually) looking into bolting ZFS/BTRFS-like partial snapshot features to simulate my own atomic distro where I am able to freely roll back if an update goes bad. Think Linux's Timeshift with something little extra.

There are downsides to adding features in layers, as opposed to integrating them with the FS, but dm can do quite a lot:

https://docs.kernel.org/admin-guide/device-mapper/snapshot.h...

DM has targets that facilitate block-level snapshots, lazy cloning of filesystems, compression, &c. Most people interact with those features through LVM2. COW snapshots are basically the marquee feature of LVM2.