Btrfs, also pronounced as 'Butter FS' or 'B-tree FS', is a copy-on-write(CoW) filesystem for Linux. It is GPL licensed and focuses on providing features like scalability, data integrity, defragmentation and fault tolerance among others. In this article, let us learn how to install and use some of the important features of btrfs.
Installation and Creation
First, install the btrfs packages if not already installed. On RedHat based distros, use:
yum install btrfs-progs
On Debian based distros, use:
apt-get install btrfs-tools
Enable the btrfs kernel module using:
modprobe btrfs
Let us now make a partition on a disk and create the btrfs filesystem on it. In the examples below, we will be using a virtual disk /dev/xvdc which does not yet have any partitions on it.
Using 'fdisk', create a new partition on the device. When prompted, choose 'n' for new partition followed by 'P' for primary and '1' for the partition number. Next, select the default values by selecting the 'Enter' key. Then select 'w' to save the changes and exit.
[root@localhost ~]# fdisk /dev/xvdc
Welcome to fdisk (util-linux 2.23.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0x4ea901ae.
The device presents a logical sector size that is smaller than
the physical sector size. Aligning to a physical sector (or optimal
I/O) size boundary is recommended, or performance may be impacted.
Command (m for help): p
Disk /dev/xvdc: 2097 MB, 2097152000 bytes, 4096000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk label type: dos
Disk identifier: 0x4ea901ae
Device Boot Start End Blocks Id System
Command (m for help): n
Partition type:
p primary (0 primary, 0 extended, 4 free)
e extended
Select (default p): p
Partition number (1-4, default 1):
First sector (2048-4095999, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-4095999, default 4095999): +1G
Partition 1 of type Linux and of size 1 GiB is set
Command (m for help): p
Disk /dev/xvdc: 2097 MB, 2097152000 bytes, 4096000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk label type: dos
Disk identifier: 0x4ea901ae
Device Boot Start End Blocks Id System
/dev/xvdc1 2048 2099199 1048576 83 Linux
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
We will now create the filesystem on the partition.
mkfs.btrfs <device>
[root@localhost ~]# mkfs.btrfs /dev/xvdc1
Detected a SSD, turning off metadata duplication. Mkfs with -m dup if you want to force metadata duplication.
Btrfs v3.16.2
See http://btrfs.wiki.kernel.org for more information.
Turning ON incompat feature 'extref': increased hardlink limit per file to 65536
fs created label (null) on /dev/xvdc1
nodesize 16384 leafsize 16384 sectorsize 4096 size 1.00GiB
Next step is to create a mount point and mount the filesystem.
[root@localhost ~]# mkdir /mnt/btrfs_mount
[root@localhost ~]# mount /dev/xvdc1 /mnt/btrfs_mount/
Verify if the mount was successful using 'df -h' command
[root@localhost ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 14G 741M 13G 6% /
devtmpfs 493M 0 493M 0% /dev
tmpfs 494M 0 494M 0% /dev/shm
tmpfs 494M 14M 481M 3% /run
tmpfs 494M 0 494M 0% /sys/fs/cgroup
/dev/xvdc1 1.0G 17M 899M 2% /mnt/btrfs_mount
Some useful mount options
Btrfs offers some special mount options which we can use to control its behaviour. To name a few,
compress [=zlib|lzo|no] - To turn on data compression using zlib or LZO compression. While zlib method is faser, LZO offers better compression
autodefrag - Enables auto defragmentation of the filesystem in the background.
nodatacow - Turn off copy-on-write for newly created files.
nodatasum - Turn off data checksum creation for new files.
The following example mounts the filesystem using lzo compression
[root@localhost ~]#mount -o compress=lzo /dev/xvdc1 /mnt/btrfs_mount
Adding and removing devices in btrfs
It is possible to add another device to the mount point.
btrfs device add <device> <mount point>
Adding a device to btrfs
Notice from the 'df -h' output above that the size of the filesystem which was 1GB earlier is now increased to 2GB.
Same way, we can also remove one or more added devices / volumes:
btrfs device delete
[root@localhost ~]# btrfs device delete /dev/xvdd1 /mnt/btrfs_mount/
[root@localhost ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 14G 786M 13G 6% /
devtmpfs 493M 0 493M 0% /dev
tmpfs 494M 0 494M 0% /dev/shm
tmpfs 494M 51M 444M 11% /run
tmpfs 494M 0 494M 0% /sys/fs/cgroup
/dev/xvdc1 1.0G 17M 899M 2% /mnt/btrfs_mount
After adding or removing devices, it is possible to redistribute the data and metadata present in the filesystem across the available devices.
btrfs filesystem balance <mount point>
[root@localhost ~]# btrfs filesystem balance /mnt/btrfs_mount
Done, had to relocate 4 out of 4 chunks
Resizing the filesystem
One can increase or decrease the size of the btrfs filesystem while it is mounted, provided the underlying device has enough space to support these actions.
btrfs filesystem resize <new size> <mount point>
Resizing the filesystem
Compression and defragmentation
Btrfs allows compression and defragmentation to increase the filesystem capacity and IO performance respectively.
Defragmentation is done using the following command:
btrfs filesystem defragment <mount point>
[root@localhost ~]# btrfs filesystem defragment /mnt/btrfs_mount
To compress and defragment parallelly,
btrfs filesystem defragment -c <mount point>
[root@localhost ~]# btrfs filesystem defragment -c /mnt/btrfs_mount
NOTE: In order to use the '-c' option, compression should be enabled at the time of mounting as specified previously in the 'Some useful mount options' section.
It is also possible to defragment files and directories individually by providing the specific files and directories.
btrfs filesystem defragment -c <file-name> ......
Subvolumes, Snapshots and Quotas
Subvolumes and snapshots are one of the key features of btrfs. A subvolume is a POSIX file namespace. It is different from LVM volumes where the volumes are block devices. We can think of them to the ones providing a restricted view of the filesystem as they can be mounted independently. Every subvolume contains a corresponding directory under its parent filesystem. We can further create other directories under this directory and even subvolumes, but its behaviour is very similar to a normal directory.
To create a subvolume, use the following command:
#btrfs subvolume create <subvolume>
[root@localhost ~]# btrfs subvolume create /mnt/btrfs_mount/subvol1
Create subvolume '/mnt/btrfs_mount/subvol1'
List the available subvolumes
#btrfs subvolume list <mount point>
[root@localhost ~]# btrfs subvolume list /mnt/btrfs_mount/
ID 259 gen 38 top level 5 path subvol1
We can try copying some data into the newly created subvolume.
[root@localhost ~]# cp /etc/hosts /mnt/btrfs_mount/subvol1
[root@localhost ~]# ls -l /mnt/btrfs_mount/subvol1/
total 4
-rw-r--r-- 1 root root 158 Aug 6 09:44 hosts
Let us now unmount the main filesystem and mount only the subvolume.
[root@localhost ~]# umount /mnt/btrfs_mount/
[root@localhost ~]# mount -o subvol=subvol1 /dev/xvdc1 /mnt
[root@localhost ~]# ls -l /mnt
total 4
-rw-r--r-- 1 root root 158 Aug 6 09:44 hosts
Notice that we are now seeing only the subvolume content. Anything above this level is not accessible, thus giving a restricted view.
Coming to snapshots, they help in taking a snapshot of the contents of subvolumes.
#btrfs subvolume snapshot
[root@localhost ~]# btrfs subvolume snapshot /mnt/btrfs_mount /mnt/btrfs_mount/snapshot1
Create a snapshot of '/mnt/btrfs_mount' in '/mnt/btrfs_mount/snapshot1'
[root@localhost ~]# ls -l /mnt/btrfs_mount
total 16
drwxr-xr-x 1 root root 14 Aug 4 09:56 snapshot1
drwxr-xr-x 1 root root 10 Aug 6 09:44 subvol1
Interestingly, a snapshot is itself a subvolume. We can use '-r' option in te above command to create a read-only snapshot.
We can delete any subvolume or snapshot using the 'btrfs subvol delete' command
[root@localhost ~]# btrfs subvol delete /mnt/btrfs_mount/subvol1
Transaction commit: none (default)
Delete subvolume '/mnt/btrfs_mount/subvol1'
[root@localhost ~]# btrfs subvol delete /mnt/btrfs_mount/snapshot1/
Transaction commit: none (default)
Delete subvolume '/mnt/btrfs_mount/snapshot1'
[root@localhost ~]# ls -l /mnt/btrfs_mount/
total 0
It is possible to limit one or more subvolumes from consuming all the filesystem space using quotas.
To use this, we need to first enable quotas on the filesystem before creating any subvolumes.
#btrfs quota enable <mount point>
[root@localhost ~]#btrfs quota enable /mnt/btrfs_mount
Now the subvolume can be limited:
#btrfs qgroup limit <size-limit> <subvolume-path>
[root@localhost ~]# btrfs subvolume create /mnt/btrfs_mount/svol1
Create subvolume '/mnt/btrfs_mount/svol1'
[root@localhost ~]# btrfs qgroup limit 500M /mnt/btrfs_mount/svol1/
Other useful options
btrfs filesystem df - Shows space usage information of a given mount point
Btrfs space usage information
btrfs filesystem show - Shows the filesystem version details
[root@localhost ~]# btrfs filesystem show /mnt/btrfs_mount/
Btrfs v3.16.2
btrfs scrub - Starts error checking on selected or all devices
[root@localhost ~]# btrfs scrub start /mnt/btrfs_mount/
scrub started on /mnt/btrfs_mount/, fsid 199b8fd8-95d4-49d5-b1b5-edc005f9bd48 (pid=20225)
Check the status of crub using the 'status' command:
[root@localhost ~]# btrfs scrub status /mnt/btrfs_mount/
scrub status for 199b8fd8-95d4-49d5-b1b5-edc005f9bd48
scrub started at Thu Aug 6 10:12:22 2015 and finished after 0 seconds
total bytes scrubbed: 288.00KiB with 0 errors
btrfs restore [options] - Restores files from a damaged filesystem without unmounting it.
For more details on btrfs, refer the btrfs wiki.
Conclusion
Though Btrfs is said to be still experimental and not fully stable, some distributions have switched to it already. It is the default file system in openSuSE 13.2 . It has many advance features and a few killer features like snapshot and rollback which makes it quite popular. Down the road, this might become the chosen filesystem for other Linux distributions as well.
The post How to Manage Btrfs Filesystem in Linux appeared first on LinOxide.