I think the main reason why btrfs has not fully replaced the ext family by now is its bugs. It's understandable that they don't want to add more buggy filesystems.
I’m completely amazed at how fast snapshots can be done, though.
Infinite memory and infinite storage are great abstractions, easy to use models and a lot of software runs on those models... but in this case it seems like ENOSPC should have been designed in from the start.
Maybe Canonical will solve that, we will see how distributing zfs together with kernel will go through.
it's been fine for some of us for, what, nearly 10 years now?
Otherwise, it is a massive pita. I do have one machine with zfs, and I made the mistake of placing the root into a subvolume. Won't do that again.
There's also FreeBSD as well as the various platforms derived from the, now defunct, OpenSolaris.
I've never had compile ZFS myself (though I have, on occasions, chosen to because I've wanted to try features before they hit the repos).
And they are supported by any linux distro out of the box.
That said, I did once run into an issue running ZFS on ArchLinux which caused data loss. That was a highly experimental set up though and was before ZoL really took off (incidentally I've also run Btrfs on ArchLinux and that also caused me data loss).
Hopefully I'm not jynxing things saying this, but ZFS has saved me from excessive downtime on a number of occasions. It has even recovered from corrupted superblock failures (when a RAID controller was faulty and randomly dropping devices during heavy load).
Non-obvious, but very straight forward way around such a wedged in file system, is to add a 3rd device. It could even be a USB stick, back then I was using small 4GiB sticks and it would work. That was enough to allocate a couple metadata only block groups to the stick, to write out the file system changes necessary to back out the second device. And once that completes, a brief filtered balance (e.g. btrfs balance start -dusage=10 is usually sufficien) allows enough free space on the 1st device, to back out the 3rd (the USB stick).
The non-obvious thing about any COW file system is that deletion always requires free space. There is no such thing as deleting a file with COW unless the fs can write that deletion change to all the affected trees into free space. Once the entire set of metadata changes is committed, then the data and metadata extents for those deleted files can be freed.
Anyway, a lot has changed even in one year in Btrfs, let alone the past five years. It's thousands of line changes per kernel release.