In a comment on yesterday's entry I was asked:

Could you elaborate on the "btrfs does not qualify" part?

What's missing? How likely do you think this to change in the near future?

I will give a simple looking answer that conceals big depths: what's missing is a btrfs webpage that doesn't say 'run the latest kernel.org kernel' and a Fedora release that doesn't say 'btrfs is still experimental and is included as a technology preview' (which is what Fedora 18 says). It's possible that btrfs is more mature and ready than I think it is, but if so the btrfs people are doing a terrible job of publicizing this. Fundamentally I want to be using something that the developers consider 'mature' or at least 'ready' and I don't want us to be among the first pioneers with a production deployment of decent size in a challenging environment.

Pragmatically there is nothing that btrfs can do to make us consider it in the near future, for reasons I wrote about two years ago in an entry on the timing of production btrfs deployments. If btrfs magically became perfect tomorrow, it would only appear in an Ubuntu LTS release in 2014 and an Red Hat Enterprise release in, well, who knows but probably not this year.

(The current Ubuntu 12.04 LTS has btrfs v3.2, whereas btrfs is up to v3.9 already. The btrfs changelog shows the scope of a year's evolution.)

As far as what in specific is missing, well, I have to confess that I haven't looked at the current state of btrfs in much detail and so I don't have specific answers. I poke at btrfs vaguely every so often; generally I discover something that strikes me as alarming and then I go away again. Since btrfs is never going to be exactly like ZFS, I can't just directly translate our our ZFS fileserver design to btrfs and then complain about what's missing or different. To have a really informed opinion on what btrfs needed and what was wrong with it, I'd have to do a btrfs-based fileserver design from scratch, trying to harmonize what we think we want (which has been shaped by what ZFS gives us) with what btrfs gives us. So far there seems to be no real point to doing that before btrfs stabilizes.

(I'm starting to think that btrfs and ZFS have fundamentally different visions about some things, but that needs some more reading and another entry.)

Sidebar: ZFS on Linux maturity versus btrfs maturity

You might ask why I'm willing to consider ZFS on Linux even though it's a relatively young project, just like btrfs. The answer is that the two are fundamentally different. The ZFS part of ZoL on Linux is generally a mature and well proven codebase; most of the uncertain new bits are just for fitting it into Linux.