What is btrfs?
The B-tree file system is a “copy-on-write (COW)” check-summed file system with multi-device support and writable snapshots.
A snapshot is an image or copy of the file system at a point in time. This feature facilitates system rollback and instant backup when combined with supporting software. Since the snapshot is writable, it can be modified.
A subvolume is like a directory - it has a name, there's nothing on it when it is created, and it can hold files and other directories. There's at least one subvolume in every Btrfs filesystem, the "default" subvolume.
The equivalent in Ext4 would be a filesystem. Each subvolume behaves as a individual filesystem. The difference is that in Ext4 you create each filesystem in a partition, in Btrfs however all the storage is in the 'pool', and subvolumes are created from the pool, you don't need to partition anything. You can create as many subvolumes as you want, as long as you have storage capacity.
Btrfs uses Rodeh's b-trees which are COW friendly. http://www.cs.tau.ac.il/~ohadrode/papers/btree_TOS.pdf
Check-summing is a data integrity method.
Multi-device support is part of the subvolume structure btrfs uses to grow or shrink a file system.
“A short history of btrfs” (LWN.NET July 22, 2009) by Valerie Aurora (formerly Henson) is available at http://lwn.net/Articles/342892/
A 45 minute video "Btrfs: Filesystem Status and Future Plans" by Chris Mason of Oracle is available at http://video.linuxfoundation.org/video/1608
More Information on btrfs is available at [[https://btrfs.wiki.kernel.org https://btrfs.wiki.kernel.org/index.php/Category:Documentation and http://en.wikipedia.org/wiki/Btrfs
Btrfs is under heavy development, but with Linux 2.6.31 the Btrfs disk format has been finalized and is not considered experimental any more. http://lkml.org/lkml/2010/6/12/66
Stability
Fedora17 to seriously consider Btrfs as default file system https://fedoraproject.org/wiki/Features/F17BtrfsDefaultFs
Initial online fsck support announced June 30, 2011. http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg10864.html
"The main concern against BTRFS is not stability, but the lack of a functional fsck utility." http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg06277.html
Update on btrfsck from Chris Mason (October 4, 2011): http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg12598.html
Update on btrfsck from Chris Mason (August 18, 2011): http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg11836.html
Btrfs wins Linux New Media Award (Chris Mason March 23, 2011). http://oss.oracle.com/~mason/btrfs/btrfs_award.jpg
Features
- Extent based file storage (2^64 max file size)
- Space efficient packing of small files
- Space efficient indexed directories
- Dynamic inode allocation
- Writable snapshots
- Subvolumes (separate internal filesystem roots)
- Object level mirroring and striping
- Checksums on data and metadata (multiple algorithms available)
- Compression
- Integrated multiple device support, with several raid algorithms
- Online filesystem check
- Very fast offline filesystem check
- Efficient incremental backup and FS mirroring
- Online filesystem defragmentation
Ubuntu-specific subvolume layout in 11.04 and later
In Ubuntu 11.04 and later, the installer sets up btrfs with a specific layout:
The default subvolume to mount is always the top of the btrfs tree (subvolid=5).
Subvolumes are created below the top of the btrfs tree as needed, e.g. for / and /home, it creates subvolumes named @ and @home. This means that specific options are needed in order to mount the subvolumes, instead of the default btrfs tree top:
The @ subvolume is mounted to / using the kernel boot option rootflags=subvol=@
The @home subvolume (if it is used), is mounted via the mount option subvol=@home in fstab.
How to work with snaphots in Ubuntu's layout
In order to work with snapshots of / or /home in the Ubuntu layout it is very convenient to mount the btrfs filesystem at a separate location, and work from the top of the btrfs tree, rather than from the mounted subvolumes.
sudo mount /dev/sdX# /mnt
Create snapshots
To create a snapshot use
sudo btrfs subvolume snapshot /mnt/@ /mnt/@_snapshot
this will create a snapshot of the @ subvolume named @_snapshot located also in the top of the btrfs tree.
Rollback to a snapshot
To roll back to a snapshot, you simply need to change it's name to the name that ubuntu mounts, using
sudo mv /mnt/@ /mnt/@_badroot sudo mv /mnt/@_snapshot /mnt/@
and reboot.
Delete a snapshot
To delete a snapshot use
sudo btrfs subvolume delete /mnt/@_badroot
btrfs snapshots are subvolumes in themselves, and self-contained, deleting the old @ subvolume like this is fine, provided we have a replacement.
The btrfs-tools command ''set-default'' will break Ubuntu's layout
Since Ubuntu is set up to always keep the top of the btrfs tree as the default mounting subvolume it will break when using the btrfs-tools command set-default, since this command is specifically designed to change the default mounting subvolume.
The mount options for / and /home described above relies on the fact that the corresponding subvolumes @ and @home can be located below the default mounting subvolume, and if set-default is used, this is no longer the case.
If you have accidentally used set-default and want to revert, you can do the following
sudo mount /dev/sdX# /mnt sudo btrfs subvolume set-default 5 /mnt
since the id 5 is a permanent alias for the top of the btrfs tree.
Fresh Install on 11.04 Natty
As of 11.04-beta1, it is possible to use only btrfs file systems with the caveat that grub _MUST_NOT_ be installed to the boot sector of the btrfs volume containing /boot. See also Ubuntu Grub2 Bug 757446 and Ubuntu Grub2 Bug 759772. You must install it to either the parent (sda rather than sda1; MBR/Reserved Sectors) _OR_ use a dedicated /boot partition as shown in the following example (in Fresh Install on 10.10 Maverick).
When installing Ubuntu in one large btrfs-Partition without an extra boot-partition, take care to keep about 1 Mib space free at the beginning of the disk. This is possible using the partition manager in the Ubuntu installer. When there is not this space, the installer fails at the end when trying to install Grub!
Fresh Install on 10.10 Maverick
Obtain the latest install cd from http://cdimage.ubuntu.com/
Do a regular installation except use manual partitioning to create:
- a ext2 or ext3 partition of about 250MB for /boot
- a btrfs partition for /
- a linux swap partition large enough to allow for suspending to RAM (amount of ram plus 100MB is a good size) If you don't need suspend support then not more than 2GB is sufficent.
Install as Root on versions prior to 10.10
An excellent HowTo is available at http://ubuntuforums.org/showpost.php?p=8716089&postcount=1
Installing on 8.10 Intrepid
PPA deb packages
The PPA specified in this community documentation is out of date, more recent packages can be found in the standard repositories.
https://launchpad.net/~brcha/+archive/ppa
Lines for sources.list.d:
deb http://ppa.launchpad.net/brcha/ppa/ubuntu intrepid main deb-src http://ppa.launchpad.net/brcha/ppa/ubuntu intrepid main
Add the GPG key:
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 17f547c39c5c4071e254d0a7313d312748a22a95
Download module source and btrfs tools:
sudo apt-get install btrfs btrfs-progs module-assistant
Build and install kernel module:
sudo module-assistant auto-install btrfs sudo modprobe btrfs
Compile from Source
NOT ACTUALLY WORKING YET
btrfs wiki getting started page
Kernel Module Dependencies
8.10 Kernel 2.6.27-9-generic
- CONFIG_LIBCRC32C=m
- CONFIG_ZLIB_INFLATE=y
- CONFIG_ZLIB_DEFLATE=m
- build-essential: make, compiler, etc
- linux-headers-2.6.27-9-generic: latest kernel headers
sudo apt-get install build-essential linux-headers-2.6.27-9-generic
Patch: http://permalink.gmane.org/gmane.comp.file-systems.btrfs/2562
Download, patch and compile code:
wget http://www.kernel.org/pub/linux/kernel/people/mason/btrfs/btrfs-0.17.tar.bz2 tar -jxvf btrfs-0.17.tar.bz2 cd btrfs-0.17 #apply patch make
Currently btrfs does not work with kernel 2.6.27
Ensure modules are up and running, and insert btrfs module into kernel.
sudo modprobe libcrc32c zlib_inflate zlib_deflate sudo insmod btrfs.ko
btrfs progs dependencies
sudo apt-get install e2fsprogs uuid-dev
wget http://www.kernel.org/pub/linux/kernel/people/mason/btrfs/btrfs-progs-0.17.tar.bz2 tar -jxvf btrfs-progs-0.17.tar.bz2 make
btrfs-progs compile fine
Demo Script
A demo of how to use btrfs features by Valerie Aurora (formerly Henson): btrfs_demo.sh
Snapshots & Subvolumes
Create a subvolume:
btrfs subvolume create test
This creates a subvolume in your home area called test. It appears to be a directory.
Do a snapshot copy of a subvolume:
btrfs subvolume snapshot test snap-copy-one
This creates a subvolume in your home area called snap-copy-one complete with all the data contained in test at the time of the snapshot.
List your subvolume:
sudo btrfs subvolume list test
This lists your subvolumes. Root privileges and the name of one of your subvolumes is required.
Sample Output:
ID 264 top level 5 path home/myhome/test ID 265 top level 5 path home/myhome/snap-copy-one
Delete your subvolume:
sudo btrfs subvolume delete test
Root privileges are required to delete subvolumes.
Adding Filesystem Compression
Edit /etc/fstab:
# <file system> <mount point> <type> <options> <dump> <pass> proc /proc proc nodev,noexec,nosuid 0 0 # / was on /dev/sda2 during installation UUID=07e198ed-18a3-41ed-9e48-bde82ead65fc / btrfs defaults, compress 0 1 # /boot was on /dev/sda1 during installation UUID=90983817-83f8-464f-a50c-39cd02317447 /boot ext2 defaults 0 2 # swap was on /dev/sda5 during installation UUID=a3da3212-876f-4350-94b3-4b5b5040c871 none swap sw 0 0
The compress option was added. Remount the filesystem using "sudo mount -o remount /" or simply reboot.
Compression algorithm:
Since kernel 2.6.38 you can choose between zlib (default) and lzo as compression algorithms. zlib has a higher compression ratio while lzo is faster and takes less cpu load. To use lzo you edit your fstab and add "compress=lzo" to your mount options as described above. Specifying "compress" equals "compress=zlib".
See Phoronix for a benchmark between zlib and lzo.
NOTE: Grub2 in 11.04 can't read lzo compressed root so you'll need a /boot partition to use it.
Display Filesystem Information
Show a device:
sudo btrfs filesystem show /dev/sda2
Displays information about device /dev/sda2.
Sample Output:
failed to read /dev/sr0
Label: none uuid: 07e198ed-18a3-41ed-9e48-bde82ead65fc
Total devices 1 FS bytes used 26.80GB
devid 1 size 912.20GB used 28.27GB path /dev/sda2
Btrfs Btrfs v0.19Show Filesystem Information for a path:
btrfs filesystem df /home/myhome
Displays information about path /home/myhome.
Sample Output:
Metadata: total=640.00MB, used=265.57MB Data: total=27.00GB, used=26.54GB System: total=12.00MB, used=12.00KB
Validate Filesystem Integrity
Fsck a device:
sudo btrfsck /dev/sda2
Does online filesystem check of device /dev/sda2.
Sample Output:
found 28752076800 bytes used err is 0 total csum bytes: 27806792 total tree bytes: 277921792 total fs tree bytes: 233484288 btree space waste bytes: 76057148 file data blocks allocated: 35377807360 referenced 28362235904 Btrfs Btrfs v0.19
Scrub a directory:
Install btrfs-scrub using instructions at http://xercestech.com/how-to-scrub-btrfs.geek
btrfs-scrub Videos
The btrfs-scrub routine reads every file in the directory Videos. Use dmesg to view errors.
Improve Filesystem Performance
Balance the B-Tree:
btrfs filesystem balance /home/myhome
In this case the Rodeh b-trees in path /home/myhome are being balanced. This could take some time.
Defragment a directory:
sudo btrfs filesystem defragment Videos
All the files in the Videos directory are defragmented. Sudo is required to get ioctl access to the files.
How to use
- How to format disk as btrfs
- How to grow/shrink file system
- How to create snapshots
- How to access snapshots
- How to convert ext2/3(/4?) partitions to btrfs
- How to create an image of a btrfs file system
- How to restore from an image of a btrfs file system
- How to recover from a failed device
- How to add another device to file system
btrfs wiki documentation category