As I am currently working with KVM KVM and I will have some big disk image files hanging around, I thought it could be a good opportunity to give a try to the btrfs snapshot and subvolume support.

First of all: snapshots and subvolumes are the same to btrfs. To be fair: snapshots are subvolumes with initial content. The nice thing about btrfs is the Copy-On-Write (COW) semantics: snapshots do not take physical space until blocks are overwritten, and only new versions of overwritten blocks are allocated from free space. This is a lot similar to KVM/Qemu snapshots when dealing with images in the qcow2 format. The difference is that you can mount a btrfs volume: you get features designed for virtualization right on the filesystem level! Once again old technology is applied in a novel way :-)

I have a btrfs volume mounted at /mnt/misc, and a 1GB Debian “Etch” image and a file with some notes about the image:

# ls -l /mnt/misc
-rw-r--r-- 1 root root 1000000000 2008-06-24 20:44 etch-base.img
-rw-r--r-- 1 root root         60 2008-06-24 18:35 etch-base.txt

Now let us create a snapshot named snappy:

# btrfsctl -s snappy /mnt/misc
# ls -l /mnt/misc
-rw-r--r-- 1 root root 1000000000 2008-06-24 20:44 etch-base.img
-rw-r--r-- 1 root root         60 2008-06-24 18:35 etch-base.txt

Hey! Wait a second! Nothing seems to happen!… This is due to all subvolumes and snapshots hanging from the filesystem root, and the default subvolume is mounted (you guessed it) by default. Let’s remount another subvolume:

# umount /mnt/misc
# mount -o subvol=snappy /dev/hda1 /mnt/misc
# ls -l /mnt/misc
-rw-r--r-- 1 root root 1000000000 2008-06-24 20:44 etch-base.img
-rw-r--r-- 1 root root         60 2008-06-24 18:35 etch-base.txt

Nothing impressive… now for the fun part we will add some garbage, and mount all the subvolumes at once:

# touch /mnt/misc/hello-btrfs.txt
# echo 'btrfs rocks' >> /mnt/misc/etch-base.txt
# umount /mnt/misc
# mount -o subvol=. /dev/hda1 /mnt/misc
# ls /mnt/misc
default  snappy

Now the mount point shows a directory for each subvolume, and the files are really there:

# ls /mnt/misc/*
/mnt/misc/default:
etch-base.img  etch-base.txt

/mnt/misc/snappy:
etch-base.img  etch-base.txt  hello-btrfs.txt

Now please run df -h /mnt/misc by yourself and get surprised: only about 1GB of physical space is really in use. In fact that can even be less than 1GB, as btrfs supports sparse files as well (if there are “gaps” they do not take physical space). Final remark: you can also make snapshots of snapshots.