Btrfs

Btrfs is a new copy on write (COW) filesystem for Linux aimed at implementing advanced features while focusing on fault tolerance, repair and easy administration. Jointly developed by Oracle, Red Hat, Fujitsu, Intel, SUSE and many others, Btrfs is licensed under the GPL and open for contribution from anyone.

Recent Developments and News Links

• Summary of Chris Mason's talk from LFCS 2012
• On 2012-03-28, btrfs-progs includes btrfsck, a tool that can fix errors on btrfs filesystems.
• Oracle has packaged this version of btrfs-progs and released it to their customers of Oracle Linux 6 and backported to 5.
• Arch Linux supplies this version in core/btrfs-progs (since version 0.19.20120328-1).

Installation

Btrfs support is included in the the linux package (as a module). User space utilities are available in btrfs-progs.

For multi-devices support (RAID like feature of btrfs) aka btrfs volume in early boot, you have to enable btrfs mkinitcpio hook (provided by mkinitcpio package) to be able to use, for example, a root btrfs volume. If the btrfs volume is a non-system volume, one can only use USEBTRFS="yes" in /etc/rc.conf

However, if you only use bare btrfs partition, such options are not needed.

For more option/feature, users may want to install mkinitcpio-btrfs^AUR from the AUR. This package will install a mkinitcpio hook called btrfs_advanced intended for those who wish to have automatic rollback feature.

Creating a Btrfs Partition

Format a New Partition to Btrfs

# mkfs.btrfs [options] dev [dev ...]

One can select multiple devices to create a RAID. Supported RAID levels include RAID 0, RAID 1 and RAID 10. By default, metadata is mirrored and data is striped.

Convert Ext3/4 to Btrfs

Warning: GRUB Legacy cannot boot with btrfs as root. Users need to install either GRUB or Syslinux. This guide assumes users are aware of this limitation.

1. Boot a live CD (Arch for example)
2. Enable [remote-core] and [remote-testing]
3. Setup the network
4. modprobe btrfs
5. Install btrfs-progs (make sure versions of dependencies match: glibc, e2fsprogs)
6. Run btrfs-convert
7. Mount the converted partition and modify the /etc/fstab file specifying either auto or btrfs for the partition type.
8. Chroot into the system and rebuild the GRUB entry (see Install from Existing Linux and GRUB articles, if unfamiliar with this procedure.

Btrfs Features

Subvolumes

One of the features of btrfs is the use of subvolumes. Subvolumes are basically a named btree that holds files and directories. They have inodes inside the tree of tree roots and can have non-root owners and groups. Subvolumes can optionally be given a quota of blocks. All of the blocks and file extents inside of subvolumes are reference counted to allow snapshotting. Similar to the dynamically expanding storage of a virtual machine that will only use as much space on a device as needed. Eliminating several half-filled partitions. One can also mount the subvolumes with different mount options giving more flexibility in security.

To create a subvolume:

# btrfs subvolume create [<dest>/]

For increased flexibility, install your system into a dedicated subvolume, and use:

rootflags=subvol=<whatever you called the subvol>

In the kernel boot parameters. It makes system rollbacks possible.

If using for the root partition, it is advisable to add crc32c to the modules array in /etc/mkinitcpio.conf as well as adding btrfs to the HOOKS.

Snapshots

To create a snapshot:

# btrfs subvolume snapshot <source> [<dest>/]<name>

Snapshots are not recursive, this means that every subvolume inside subvolume will be an empty directory inside the snapshot.

Defragmentation

Btrfs supports online defragmentation. To defragment the metadata of the root folder, simply do:

# btrfs filesystem defragment /

This will not defragment the entire system. For more information, see this page on the btrfs wiki.

Compression

Btrfs supports transparent compression, which means every file on the partition is automatically compressed. This does not only reduce the size of those files, but also improves performance, in particular if using the lzo algorithm. Compression is enabled using the compress=gzip or compress=lzo mount options. Only files created or modified after the mount option is added will be compressed, so to fully benefit from compression it should be enabled during installation. After preparing the hard drive, simply switch to another terminal (Template:Keypress), and run the following command:

# mount -o remount,compress=lzo /dev/sdXY /mnt/target

After the installation is finished, add compress=lzo to the mount options of the root filesystem in /etc/fstab.

Resources

• Btrfs Wiki
• BTRFS Problem FAQ - Official FAQ
• Funtoo Btrfs wiki entry - Very well-written article