Linux kernel version 2.6.29 introduced the Btrfs file system.
Btrfs—short for B-tree file system—was accepted into the Linux kernel mainline in 2009 and first appeared with kernel 2.6.29, released on 23 March. Its design aimed to provide modern features that were difficult to add cleanly to older filesystems, including copy-on-write storage, snapshots, checksums, transparent compression, and flexible multi-device management.
The underlying B-tree ideas had been discussed in earlier research. IBM researcher Ohad Rodeh proposed a copy-on-write B-tree design in 2007, and Chris Mason, then associated with SUSE and later Oracle, began developing a Linux filesystem based on related concepts. Btrfs was initially treated cautiously, with several distributions offering it as an experimental installation choice.
The version number is easy to confuse with nearby releases. Kernel 2.6.28 introduced stable ext4 support, while 2.6.29 is the release associated with Btrfs support. Btrfs later became the default filesystem for Fedora desktop variants beginning with Fedora 33 in 2020.