I recently have some issue with docker and btrfs which makes me want to learn more about it. The reason I have used btrfs is to solve some issues with device mapper.

I use docker with btrfs. Recently, for some reasons the /var/lib/docker grow to 100% disk space and docker unable to start with some error about opening db file or so.

I tried to take a snapshot of EBS volume and resize it with resize2fs as I always do when resizing volume on EBS. But turning out that doesn’t work due to btrfs.

Luckily I found an much nicer way to increase. Simply add a new volume, then using this:

sudo btrfs device add -f /dev/xvdc /var/lib/docker

This adds extra space to /var/lib/docker. At least, this will make docker start backup and I can backup my docker image or inspect/debug some existing container.