A typical Unix-style filesystem contains a single directory tree with a single root. ( / ) By default, a BTRFS filesystem is organized in the same way. Subvolumes change that picture by creating alternative roots that function as independent filesystems in their own right.
home$ sudo btrfs subvolume create home-backup
[sudo] password for:
Create subvolume './home-backup'
A subvolume looks like any other folder but event though it looks like an ordinary subdirectory, the filesystem treats it as if it were on a separate physical device.
~$ sudo ln 1395287298-iptables.save ../home-backup/
ln: failed to create hard link ‘../home-backup/1395287298-iptables.save’ => ‘1395287298-iptables.save’: Invalid cross-device link
We can mount it independently.
home$ sudo btrfs subvolume list /home/
ID 370 gen 59390 top level 5 path home-backup
home$ sudo mount -o subvolid=370 /dev/sdb7 /mnt/2
SnapshotsA snapshot in Btrfs is a special type of subvolume — one which contains a copy of the current state of some other subvolume.
BTRFS subvolume quotas, track and regulate usage by subvolumes, with no regard for the ownership of the files that actually take up the space. The other thing worth bearing in mind is that the quota mechanism is relatively new. The management tools are on the rudimentary side, there seem to be some performance issues associated with quotas, and there's still a sharp edge or two in there waiting for unlucky users.