Difference between revisions of "Btrfs"

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btrfs works in block-pairs for raid0, raid1, and raid10. This means:
btrfs works in block-pairs for raid0, raid1, and raid10. This means:
raid0 - block-pair written to 2 devices
raid0 - block-pair written to 2 devices<br>
raid1 - block-pair stripped across 2 devices
raid1 - block-pair stripped across 2 devices

Revision as of 19:37, 24 April 2013

zh-CN:Btrfs Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary end

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


Btrfs support is included in 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. However, if you only use a bare btrfs partition, this hook is not necessary.

For more options/features, users may want to install mkinitcpio-btrfsAUR 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

Warning: Grub 2 can boot btrfs partitions, but the module is larger than e.g. ext4. This means that the core.img file made by grub-install might not fit between the MBR and the first partition. You can solve this by using GPT or simply putting an extra 1 or 2 MB of free space before the first partion.

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. Run btrfs-convert <root-partition>
  3. Mount the converted partition and modify the /etc/fstab file specifying either auto or btrfs for the partition type. You should also set the fs_passno parameter (the last field) to 0 since btrfs does not require a filesystem check on every boot.
  4. Chroot into the system and rebuild the GRUB entry (see Install from Existing Linux and GRUB articles, if unfamiliar with this procedure).

After btrfs-convert, and confirming everything works, if you want to make the change permanent, you need to delete the saved image, delete the subvolume that image is on, then balance the drive to reclaim the space. For example, after converting /home to btrfs:

rm /home/ext2_saved/*
btrfs subvolume delete /home/ext2_saved


Btrfs has no built-in encryption support (this may come in future), but you can encrypt the partition before running mkfs.btrfs. See Dm-crypt with LUKS.

(If you've already created a btrfs file system, you can also use something like EncFS or TrueCrypt, though perhaps without some of btrfs' features.)

Btrfs Features

Copy-On-Write (CoW)

CoW comes with some advantages, but can negatively affect performance with large files that have small random writes. It is recomended to disable CoW for database files and virtual machine images. You can disable CoW for the entire block device by mounting it with "nodatacow" option. However, this will disable CoW for the entire file system. To disable CoW for single files/directories, use the following command:

# chattr +C [file/directory path]

Note, from chattr man page: For btrfs, the 'C' flag should be set on new or empty files. If it is set on a file which already has data blocks, it is undefined when the blocks assigned to the file will be fully stable. If the 'C' flag is set on a directory, it will have no effect on the directory, but new files created in that directory will have the No_COW attribute.

Multi-device filesystem and RAID feature

Multi-device filesystem

When creating a btrfs filesystem, you can pass as many partitions or disk devices as you want to mkfs.btrfs. The filesystem will be created across these devices. You can "merge" this way, multiple partitions or devices to get a big btrfs filesystem.

You can also add or remove device from an existing btrfs filesystem (caution is mandatory).

A multi-device btrfs filesystem (also called a btrfs volume) is not recognized until

 # btrfs device scan

has been run. This is the purpose of the btrfs mkinitcpio hook or the USEBTRFS variable in /etc/rc.conf

RAID features

When creating multi-device filesystem, you can also specify to use RAID0, RAID1 or RAID10 across the devices you have added to the filesystem. RAID levels can be applied independently to data and meta data. By default, meta data is duplicated on single volumes or RAID1 on multi-disk sets.

btrfs works in block-pairs for raid0, raid1, and raid10. This means:

raid0 - block-pair written to 2 devices
raid1 - block-pair stripped across 2 devices

For 2 disk sets, this matches raid levels as defined in md-raid (mdadm). For 3+ disk-sets, the result is entirely different than md-raid.

For example:
3 1TB disks in an md based raid1 yields a /dev/md0 with 1TB free space and the ability to safely loose 3 disks without losing data. 3 1TB disks in a btrfs volume with data=raid1 will allow the storage of approximately 1.5TB of data before reporting full. Only 1 disk can safely be lost without losing data.

btrfs uses a round-robin scheme to decide how block-pairs are spread among disks. As of Linux 3.0, a quasi-round-robin scheme is used which prefers larger disks when distributing block pairs. This allows raid0 and raid1 to take advantage of most (and sometimes all) space in a disk set made of multiple disks. For example, a set consisting of a 1TB disk and 2 500GB disks with data=raid1 will place a copy of every block on the 1TB disk and alternate (round-robin) placing blocks on each of the 500GB disks. Full space utilization will be made. A set made from a 1TB disk, a 750GB disk, and a 500GB disk will work the same, but the filesystem will report full with 250GB unusable on the 750GB disk. To always take advantage of the full space (even in the last example), use data=single. (data=single is akin to JBOD defined by some raid controllers)


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. This is 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, in the kernel boot parameters, use:

rootflags=subvol=<whatever you called the subvol>

This makes system rollbacks possible.

If using for the root partition, it is advisable to add crc32c (or crc32c-intel for Intel machines) to the modules array in /etc/mkinitcpio.conf as well as adding btrfs to the HOOKS.


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.


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. To defragment the entire system verbosely, try instead:

# find / -xdev -type f -print -exec btrfs filesystem defrag '{}' \;


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 storage 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.