Debian lets you format the HD as btrfs at the installation stage, but of course it then creates the entire posix tree as directories, not subvolumes. The Debian installer does not offer any option to create btrfs subvolumes.

Assuming I already have a system running on btrfs, is it possible to migrate the whole thing to subvolumes, including /etc, /usr, /tmp etc?

What about the very top of the tree, so I can have snapshots of the entire tree? Is that possible?

TIA

The usual way is to create what you need on another distro and then try to install over with other distro.

"The DebianInstaller can format and install to single-disk Btrfs volumes, but does not yet support multi-disk btrfs volumes nor subvolume creation (Bug #686097). Daniel Pocock has a good article on how to Install Debian wheezy and jessie directly with btrfs RAID1; however, strictly speaking it showcases Btrfs' integrated multi-device flexibility. eg: Install to a single disk, add a second disk to the volume, rebalance while converting all data and metadata to raid1 profile. "

This is the page you need to start at and follow the few links. https://wiki.debian.org/Btrfs

You are trading a fully supported ext4 filesystem for an almost fully supported filesystem. I'd say one ought to consider using the default filesystem at least for the basic distro install and then use things like zfs,btrfs and such for data.

Sorry for the tangent, but arguably your data is valuable, whereas the operating system can be recreated quickly. Thus my opinion is the opposite: Put root on BTRFS, and keep your data on a robust, well-tested filesystem. That's how OpenSUSE does it, too. It has the additional advantage that you can use subvolume snapshots to quickly go back to a previous operating system version after a failed update.

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I agree on that wholeheartedly. However, BTRFS is stable enough. I've been using it for two and a half years without a hitch, in spite of prolonged abuse. I had to use for months an application that caused my system to freeze and forced me to hard reset the machine many times. Never lost a file. I lost a few files on two occasions of hard reset with ext3 about 10 years ago. I'd rather rely on backups than on file system stability, and the snapshot feature is useful.

I've been using it for well over twice as long. Almost exclusively for my photos - and I couldn't work without snapshot. For general system usage I still think the promise is better than the reality. Look at the contortions snapper (and hence opensuse) goes to - boot roll-backs after an (system) update certainly has appeal.

As for moving to all subvolumes, see the opensuse or Arch doco - not sure how it will co-operate with the Debian mindset tho'.