How does docker map host partitions? - linux

Question

I'm relatively new to docker, and when I started a container (an ubuntu base image), I noticed the following:
On the host,
$ df -h
...
/dev/sdc1 180M 98M 70M 59% /boot
/dev/sdc2 46G 20G 24G 46% /home
/dev/sdc5 37G 7.7G 27G 23% /usr
/dev/sdc6 19G 13G 5.3G 70% /var
$ lsblk
...
sdc 8:32 0 232.9G 0 disk
├─sdc1 8:33 0 190M 0 part /boot
├─sdc2 8:34 0 46.6G 0 part /home
├─sdc3 8:35 0 18.6G 0 part /
├─sdc4 8:36 0 1K 0 part
├─sdc5 8:37 0 37.3G 0 part /usr
├─sdc6 8:38 0 18.6G 0 part /var
├─sdc7 8:39 0 29.8G 0 part [SWAP]
└─sdc8 8:40 0 42.8G 0 part
On the container
$ df -h
Filesystem Size Used Avail Use% Mounted on
rootfs 19G 13G 5.3G 70% /
none 19G 13G 5.3G 70% /
tmpfs 7.8G 0 7.8G 0% /dev
shm 64M 0 64M 0% /dev/shm
/dev/sdc6 19G 13G 5.3G 70% /etc/hosts
tmpfs 7.8G 0 7.8G 0% /proc/kcore
tmpfs 7.8G 0 7.8G 0% /proc/latency_stats
tmpfs 7.8G 0 7.8G 0% /proc/timer_stats
$ lsblk
sdc 8:32 0 232.9G 0 disk
|-sdc1 8:33 0 190M 0 part
|-sdc2 8:34 0 46.6G 0 part
|-sdc3 8:35 0 18.6G 0 part
|-sdc4 8:36 0 1K 0 part
|-sdc5 8:37 0 37.3G 0 part
|-sdc6 8:38 0 18.6G 0 part /var/lib/cassandra
|-sdc7 8:39 0 29.8G 0 part [SWAP]
`-sdc8 8:40 0 42.8G 0 part
Question 1: why is sdc6 mounted on different places between the host and the container?
Because the contents of the two mount points are different, so I assume docker must have done some kind of device mapping on the container, so sdc6 in the container isn't the same as the one on the host. However, the partition capacity and usage are the same, so I'm confused here.
Question 2: why is the container's root dir usage so high? The docker image doesn't have much stuff on it.
Thanks for any help.
Addition
The Dockerfile has a line
VOLUME /var/lib/cassandra

Answer 1

Question 1: why is sdc6 mounted on different places between the host and the container?
/dev/sdc6 on your host is /var, which is where /var/lib/docker resides and where Docker keeps certain data, such as the hosts file allocated to your container.
The hosts file is exposed as a bind mount inside the container, which is why you see:
/dev/sdc6 19G 13G 5.3G 70% /etc/hosts
Question 2: why is the container's root dir usage so high? The docker image doesn't have much stuff on it.
Take a look at the df output inside the container:
rootfs 19G 13G 5.3G 70% /
Now look at the df output on your host, and you'll see:
/dev/sdc6 19G 13G 5.3G 70% /var
The df inside the container is reflecting the state of the host filesystem. This suggests that you are using the aufs or overlay storage driver, both of which create "overlay" filesystems for containers on top of the host filesystem. The output from df would look different if you were using the devicemapper storage driver, relies on device mapper block devices instead of overlay filesystems.