Session 1 · Why containers? Virtual machines versus containers#

The big idea#

Software works on your laptop and breaks on the server because the two machines differ: another Python version, a missing library, a different setting. A container packs your program together with everything it needs into one unit that runs the same everywhere. It is lighter than a virtual machine because containers share the host’s operating-system kernel instead of each carrying a whole OS.

Everyday example: the grand house. Many families once shared one big house: one kitchen, one bathroom, constant clashes over the stove. Giving every family its own separate house (a virtual machine) ended the fights but cost a lot, because each house needs its own foundation, roof and plumbing. Apartments in one building (containers) give each family a private flat with its own door and its own kitchen, while sharing the foundation and the pipes. Cheaper, quicker to build, still private.

Each virtual machine carries its own operating system. Containers share the host's kernel, so they are smaller and start faster.

Proof: the container uses the host’s kernel#

Below, the same command runs on the host and inside an Alpine Linux container. The kernel version is identical, because there is only one kernel. Yet the distribution inside is Alpine, because the container brings its own files and libraries.

echo '--- host kernel'
uname -r
echo '--- kernel seen inside an alpine container'
docker run --rm alpine uname -r
echo '--- which Linux distribution is inside?'
docker run --rm alpine head -1 /etc/os-release
echo '--- image size'
docker images alpine --format '{{.Repository}}:{{.Tag}}  {{.Size}}'
echo '--- how long does a container take to start, run and exit?'
TIMEFORMAT='real %3Rs'
time docker run --rm alpine echo "hello from a container"

--- host kernel
6.8.0-138-generic
--- kernel seen inside an alpine container
6.8.0-138-generic
--- which Linux distribution is inside?
NAME="Alpine Linux"
--- image size
alpine:latest  8.42MB
alpine:3.20  7.81MB
--- how long does a container take to start, run and exit?
hello from a container
real 0.267s

What this shows:

Host and container report the same kernel (6.8.0-138-generic on this machine). A VM would report its own.
The container is a separate Alpine userland (NAME="Alpine Linux") even though the host is something else. That is the isolation: its own files, its own libraries.
The whole Alpine image is under 9 MB. A typical VM image is gigabytes.
Starting, running and removing a container took about a quarter of a second on this machine (your time will differ). Booting a VM takes seconds to minutes.

When to choose which#

	Virtual machine	Container
Isolation	Strong (own OS)	Good (shared kernel)
Size	Gigabytes	Megabytes
Start time	Seconds to minutes	Under a second
Run a different OS (Windows on Linux)	Yes	No, same kernel as the host
Typical use	Whole servers, strong separation	Apps and services, many per machine

They are not rivals: cloud providers often run containers inside VMs.

Common mistakes#

“A container is a small VM.” It is not. It is an ordinary process on the host, fenced off so it sees only its own files, processes and network.
Expecting a Windows container on a Linux host (or the reverse). Containers share the host kernel, so the OS family must match.
Thinking containers are automatically secure. A shared kernel means a kernel bug can matter more than in a VM. Don’t run untrusted code as root.
Treating a container like a pet server. Don’t log in and tweak it by hand; rebuild it from a recipe (the Dockerfile, Session 7).

Hands-on exercises#

Exercise 1. Run docker run --rm alpine uname -a and uname -a on your host. What is the same and what is different?

Solution

The kernel release (e.g. 6.8.0-138-generic) is identical, because they share the kernel. The hostname differs: the container gets a random id as its hostname.

Exercise 2. Which of these needs a VM and which can use a container: (a) running a Windows-only app on a Linux server, (b) running five copies of a Flask API side by side?

Solution

(a) needs a VM (or a Windows host), because the Windows kernel is needed. (b) is ideal for containers: five identical, light, quickly started copies.

Exercise 3. Find out how large the alpine image is on your machine and compare it with any VM image file you have.

Solution

docker images alpine --format '{{.Repository}}:{{.Tag}}  {{.Size}}'

On this machine alpine:latest was 8.42MB. A VM disk image is usually several GB, hundreds of times larger.

Practice#

Take the Session 1 quiz. The original story page is kept in the reference section as Virtual Machine and The Grand House.