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There have been quite a few questions about how to accomplish ZFS tasks in Btrfs, as well as just generic "How do I do X in Btrfs?". This page aims to answer those questions. Simply add your question to the "Use Case" section of the table.
1.1 How do I create a RAID1 mirror in Btrfs?
1.2 How do I create a RAID10 striped mirror in Btrfs?
1.3 How can I create a RAID-1 filesystem in "degraded mode"?
1.4 How do I determine the raid level currently in use?
1.5 How do I change RAID levels?
2.1 I want to be able to do rollbacks with Btrfs
2.3 How do I mount the real root of the filesystem once I've made another subvolume the default?
2.4 Can a snapshot be replaced atomically with another snapshot?
2.6 How I can know how much space is used by a volume?
2.8 Can we create virtual block device in BTRFS?
2.9 How do we implement quota in BTRFS?
2.10 Can I take a snapshot of a directory?
3.1 How do I label a filesystem?
3.2.3 Set the FS size
3.2.4 Resize on multi devices filesystem
3.3 How do I defragment many files?
3.5 What is best practice when partitioning a device that holds one or more btr-filesystems
3.6 How do I copy a large file and utilize COW to keep it from actually being copied?
See Using Btrfs with Multiple Devices for a guide to the RAID features of Btrfs.
Some high-level concepts are also explained in the SysadminGuide.
How do I create a RAID1 mirror in Btrfs?
For more than one device, simply use
btrfs fi df /mount Data, RAID1: total=1.00GB, used=128.00KB Data: total=8.00MB, used=0.00 System, RAID1: total=8.00MB, used=4.00KB System: total=4.00MB, used=0.00 Metadata, RAID1: total=1.00GB, used=24.00KB Metadata: total=8.00MB, used=0.00
NOTE This does not do the 'usual thing' for 3 or more drives. Until "N-Way" (traditional) RAID-1 is implemented: Loss of more than one drive might crash the array. For now, RAID-1 means 'one copy of what's important exists on two of the drives in the array no matter how many drives there may be in it'.
How do I create a RAID10 striped mirror in Btrfs?
mkfs.btrfs -m RAID10 -d RAID10 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
How can I create a RAID-1 filesystem in "degraded mode"?
Kernel 3.3 onwards - no need! Just create a conventional btrfs filesystem, then later when you want to add a second device and convert to RAID1, do this via Balance Filters
For earlier kernel versions: Create a filesystem with a small, empty device, and then remove it:
dd if=/dev/zero of=/tmp/empty bs=1 count=0 seek=4G losetup /dev/loop1 /tmp/empty mkfs.btrfs -m raid1 -d raid1 /dev/sda1 /dev/loop1 losetup -d /dev/loop1 rm /tmp/empty
You can add in the real mirror device(s) later:
mount -odegraded /dev/sda1 /mnt btrfs dev add /dev/sdb1 /mnt
(kernel 2.6.37) Simply creating the filesystem with too few devices will result in a RAID-0 filesystem. (This is probably a bug). (kernel 3.2.0-31 [ubuntu] Unfortunately btrfs seems unwilling to allocate additional block groups while mounted in degraded mode; maybe this is changed more recently)
How do I determine the raid level currently in use?
On a 2.6.37 or later kernel, use
btrfs fi df /mountpoint
The required support was broken accidentally in earlier kernels, but has now been fixed.
How do I change RAID levels?
From kernel 3.3 onwards, it is possible to change the raid level of data and metadata using the btrfs balance command, with the convert= filter.
I want to be able to do rollbacks with Btrfs
To roll back a subvolume to an earlier one, you simply need to replace one with the other. There are several ways of achieving this, depending on where you have the subvolumes, and how they're accessed (mounted or not mounted).
If your snapshots are kept alongside the original subvolume, say:
/media/btrfs |--- @home `--- snapshots `--- @home-20170105
then you can simply rename the snapshots:
# cd /media/btrfs # mv @home @home.old # mv snapshots/@home-20170105 @home
If you want to keep the snapshot, or it is read-only, make another read/write snapshot instead:
# cd /media/btrfs # mv @home @home.old # btrfs sub snap snapshots/@home-20170105 @home
After doing either of these, you will need to unmount and remount the subvol:
# umount /home # mount /home
Obviously, if this is your root subvol (mounted at /), then you will have to reboot in order to do this last step.
If your / is also the top level of your filesystem, then you can't atomically roll back to an earlier snapshot. You should definitely consider reorganising your filesystem to one of the recommended patterns.
How can I use btrfs for backups/time-machine?
(See also the page on Incremental Backup).
Snapper — tool for managing btrfs snapshots, with ability to compare snapshot differences or revert changes. Originally developed for openSUSE, nowadays spreads it use among other distros (Debian, Ubuntu, Fedora, Mandriva), available also for enterprise distros (SLES, RHEL, CentOS).
btrfs-time-machine — save any mounted filesystem to a btrfs filesystem with per-date snapshots. You can configure it to do a temporary source-side snapshot when backing up btrfs filesystems, and you can enable deduplication on the destination.
UrBackup - save backups as btrfs sub-volumes. Incremental backups are created using btrfs snapshots. Blocks which are not changed in files in incremental backups are stored only once using cross-device reflinks. See File Backup Storage with Btrfs Snapshots for details.
rsyncbtrfs — simple bash script which uses rsync to perform incremental btrfs backup, creating subvolumes automatically
btrfs-backup — Another, very small (50 lines), rsync script intended as base for extensions and ideas. It implements a custom 5-level rotation scheme based on snapshot date and time.
There is a proposal for somehow generic solution, called Autosnap (initial proposal, patch).
A simple script which could be tuned for your needs.
A python script called SnapBtr.
Extension of SnapBtr called snapbtrex with btrfs send and receive capabilities for remote backup sync [1]
How do I mount the real root of the filesystem once I've made another subvolume the default?
With kernel 3.2 and newer you can specify subvol=/some/PATH for the subvolume to mount
mount -o subvol=/path/to/subvol /dev/sdx /mnt
The PATH is always relative to the toplevel subvolume, ie. independent of currently set default subvolume.
Can a snapshot be replaced atomically with another snapshot?
btrfs subvolume snapshot first second
creates a snapshot of the subvolume first. After changing second in some way, I'd like to replace first with second e.g. using
btrfs subvolume snapshot second first
This isn't currently allowed. I would need to delete first before snapshotting, but this leaves a period of time when there is no directory by that name present, hence the need for atomic replacement á la rename(2).
Is this possible with current btrfs?
No, and it's going to be pretty much impossible to achieve. You would have to ensure that all users of the volume being replaced have stopped using it before you replaced it. If you're going to do that, you might as well do an unmount/mount.
Are there a similar commands in BTRFS like ZFS send/receive?
Btrfs also has send/receive functionality that works on the level of subvolume snapshots.
When a read only snapshot of a subvolume is taken, it can be transferred to another location, using the `btrfs send` and `btrfs receive` commands.
After taking subsequent read only snapshots of the same subvolume, `btrfs send` can efficiently send changes since the previous snapshot incrementally, even when working with large file trees.
Since `btrfs send` and `btrfs receive` are separate processes, the data stream that is generated by send can also be stored in any intermediate format, and be fed to the receive process later.
How I can know how much space is used by a volume?
This is a complex question. Since snapshots are subvolumes, storage can be shared between subvolumes, so how do you assign ownership of that storage to any specific subvolume?
It does make sense to ask "how much space would be freed if I deleted this subvolume?", but this is expensive to compute (it will take at least as long as "du" to run), and currently there is no code that will do the calculation for you.
Are there a similar commands in BTRFS like ZFS export/import?
Moving a Btrfs filesystem to another computer works equal to moving any other file system on Linux. You umount it, move the disk to another system, make sure it sees the disk and then mount the file system again.
When a filesystem consists of multiple block devices added together, all of them should be present, of course.
Can we create virtual block device in BTRFS?
No. Btrfs doesn't support the creation of virtual block devices or other non-btrfs filesystems within its managed pool of space.
If you use file-backed storage for virtual machines, set those files nocow with `chattr +C`, see FAQ#Can copy-on-write be turned off for data blocks?. Otherwise you will get poor performance and fragmentation[2].
How do we implement quota in BTRFS?
Once a BTRFS has been created, quota must be enabled before any subvolume is added using btrfs quota enable  . If quotas weren't enabled, you must first enable them, then create a qgroup (quota group) for each of those subvolume using their subvolume ID and rescan them, for example;
btrfs subvolume list   | cut -d' ' -f2 | xargs -I{} -n1 btrfs qgroup create 0/{}
Rescan the subvolumes (added in kernel 3.11)
Then you can assign a limit to any subvolume using;
btrfs qgroup limit 100G  /
You can look at quota usage using
The original document discussing the concepts and implementation is at http://sensille.com/qgroups.pdf
A more descriptive listing of the commands is shown in this patch.
A proposal being discussed (2011/06) http://comments.gmane.org/gmane.comp.file-systems.btrfs/11095
Disk structure proposal (2011/07) http://permalink.gmane.org/gmane.comp.file-systems.btrfs/11845
Can I take a snapshot of a directory?
You can't, only subvolumes can be really snapshotted. However, there are ways how to achieve a very similar result, provided that cross-subvolume reflinks are allowed (discussion about this feature -- not in 3.3 as of writing; merged since 3.6 [3]).
Suppose there's a source directory src, and you want to take a snapshot of this directory named dest, then possibly creating more snapshots of it:
btrfs subvolume create dest cp -ax --reflink=always src/. dest
Note, that the syntax src/. is important, this will copy contents of src rather creating the dir src under dest. Now the directories hold the same files and data, but any change is independent due to nature of reflink.
Speed of the reflink copy is decent. After creating the subvolume, you can use it as any other subvolume, more specifically you can do the real snapshots very quickly now!
How do I label a filesystem?
The label on a filesystem can be set:
mkfs.btrfs -L LABEL /dev/sdx
on a mounted and unmounted filesystem
btrfs filesystem label /dev/sdx [newlabel] btrfs filesystem label /path [newlabel]
How do I resize a partition? (shrink/grow)
In order to demonstrate and test the back references, Btrfs devel team has added an online resizer, which can both grow and shrink the filesystem via the btrfs commands.
First, ensure that your filesystem is mounted. See elsewhere for the full list of btrfs-specific mount options
mount -t btrfs /dev/xxx /mnt
Enlarge the filesystem by 2 GiB:
btrfs filesystem resize +2G /mnt
The parameter "max" can be used instead of (e.g.) "+2G", to grow the filesystem to fill the whole block device.
To shrink the filesystem by 4 GiB:
btrfs filesystem resize -4g /mnt
Set the FS size
To set the filesystem to a specific size, omit the leading + or - from the size. For example, to change the filesystem size to 20 GiB:
btrfs filesystem resize 20g /mnt
Resize on multi devices filesystem
To affect a specific device of a multi devices filesystem add "devid:"
# btrfs filesystem show Label: 'RootFS' uuid: c87975a0-a575-425e-9890-d3f7fcfbbd96 Total devices 2 FS bytes used 284.98GB devid 2 size 311.82GB used 286.51GB used 286.51GB path /dev/sdb1 devid 1 size 897.76GB used 286.51GB path /dev/sda1 # btrfs filesystem resize 2:max /mnt/RootFS Resize '/mnt/RootFS' of '2:max' # btrfs filesystem show Label: 'RootFS' uuid: c87975a0-a575-425e-9890-d3f7fcfbbd96 Total devices 2 FS bytes used 284.98GB devid 2 size 897.76GB used 286.51GB path /dev/sdb1 devid 1 size 897.76GB used 286.51GB path /dev/sda1
How do I defragment many files?
Since version v3.12 of btrfs progs it is possible to run a recursive defragmention using:
sudo btrfs filesystem defragment -r /path/
Prior to this version one would use:
sudo find [subvol [subvol]…] -xdev -type f -exec btrfs filesystem defragment -- {} +
This won't recurse into subvolumes (-xdev prevents recursion into non-btrfs filesystems but also btrfs subvolumes).
You may also want to set the non-default autodefrag mount option.
Caveat: since Linux 3.14-rc2, 3.13.4, 3.12.12 and 3.10.31 (which removed the snapshot-aware defragmentation) defragmenting a file which has a COW copy (either a snapshot copy or one made with cp --reflink or bcp) would produce two unrelated files. If you defragment a subvolume that had a snapshot, you would roughly double the disk usage, as the snapshot files would no longer be COW images of the originals.
One would get this rather unexpected outcome when using defragment (together with "-c") to compress files with the hope of saving space.
Note that this is the same behaviour as observed with any pre 3.9 kernel (which added snapshot-aware defragmentation).
ZFS does not need file system entries in /etc/(v)fstab. Is it mandatory in BTRFS to add file system entries in /etc/fstab?
ZFS auto-mounts pools and filesystem trees. It does not require entries in /etc/fstab in order to tell the system where subtrees should be mounted.
btrfs does not implement this feature directly. However, since btrfs subvolumes are visible within and navigable from their parent subvolume, you can mount a whole tree containing many embedded subvolumes by simply mounting the top-level tree.
What is best practice when partitioning a device that holds one or more btr-filesystems
Do it the old fashioned way, and create a number of partitions according to your needs? Or create one big btrfs partition and use subvolumes where you would normally create different partitions?
What are the considerations for doing it either way?
How do I copy a large file and utilize COW to keep it from actually being copied?
You may want to make copies of a large file (like a virtual machine) within a volume/subvolume and have those copies behave as independent files (as opposed to traditional hard-links) when edited but not need to store common blocks multiple times. To do this, you can use cp's built in --reflink option. By specifing --reflink=always you are guaranteed that it will take advantage of btrfs's COW (Copy On Write) functionality. If you use --reflink=always on a non-COW capable filesyste, you will be given an error.