Btrfs (B-tree file system, variously pronounced "Butter F S", "Butterfuss", "Better F S", or "B-tree F S") is a GPL-licensed experimental copy-on-write file system for Linux. Development began at Oracle Corporation in 2007. It is still in heavy development and marked as unstable. Especially when the filesystem becomes full, no-space conditions arise which might make it challenging to delete files.
Btrfs is intended to address the lack of pooling, snapshots, checksums and integral multi-device spanning in Linux file systems, these features being crucial as Linux use scales upward into the larger storage configurations common in enterprise. Chris Mason, the principal Btrfs author, has stated that its goal was "to let Linux scale for the storage that will be available. Scaling is not just about addressing the storage but also means being able to administer and to manage it with a clean interface that lets people see what's being used and makes it more reliable."
In 2008, the principal developer of the ext3 and ext4 file systems, Theodore Ts'o, stated that although ext4 has improved features, it is not a major advance, it uses old technology, and is a stop-gap; Ts'o believes that Btrfs is the better direction because "it offers improvements in scalability, reliability, and ease of management". Btrfs also has "a number of the same design ideas that reiser3/4 had".