Containers vs virtual machines in Docker - Quick Revision & Key Differences

Recall & Review

beginner

What is a container in simple terms?

A container is like a small, portable box that holds an app and everything it needs to run, sharing the computer's main system but staying separate from other apps.

beginner

How does a virtual machine (VM) differ from a container?

A VM is like a whole computer inside your computer, with its own operating system, while a container shares the main operating system but keeps apps isolated.

intermediate

Why are containers usually faster to start than virtual machines?

Containers start faster because they share the main operating system and don't need to boot a full separate system like VMs do.

intermediate

What is one main advantage of virtual machines over containers?

Virtual machines provide stronger isolation because each VM runs a full operating system, making them better for running different OS types or very secure environments.

intermediate

Can containers run different operating systems than the host machine?

No, containers share the host's operating system kernel, so they cannot run a different OS than the host, unlike virtual machines which can.

What do containers share with the host system?

A The entire operating system

B The operating system kernel

C Hardware only

D Nothing, they are fully separate

Containers share the host's operating system kernel but keep applications isolated.

Which technology boots a full operating system inside your computer?

A Virtual machine

B Container

C Docker image

D Kubernetes pod

Virtual machines boot a full operating system, unlike containers.

Why might you choose a virtual machine over a container?

A For faster startup times

B To save disk space

C To run a different operating system than the host

D Because they share the host OS kernel

Virtual machines can run different operating systems than the host, containers cannot.

Which is generally more lightweight and uses fewer resources?

A Containers

B Virtual machines

C Both use the same resources

D Neither, they are the same

Containers are more lightweight because they share the host OS kernel.

What is a key security difference between containers and virtual machines?

A Containers have stronger isolation

B Neither provides isolation

C Both have the same isolation level

D Virtual machines have stronger isolation

Virtual machines provide stronger isolation by running separate OS instances.

Explain the main differences between containers and virtual machines in simple terms.

Think about how each runs apps and uses the computer's system.

Describe a situation where you would prefer using a virtual machine instead of a container.

Consider when full separation from the host system is important.

Practice

1. Which statement best describes the main difference between containers and virtual machines?

easy

A. Containers require more disk space than virtual machines.

B. Virtual machines are faster to start than containers.

C. Virtual machines share the host OS kernel, containers do not.

D. Containers share the host OS, while virtual machines run separate full OS instances.

Solution

1. Step 1: Understand container architecture

   Containers share the host operating system kernel, making them lightweight and fast.

2. Step 2: Understand virtual machine architecture

   Virtual machines run a full guest OS on top of a hypervisor, making them heavier and fully isolated.

3. Final Answer:

   Containers share the host OS, while virtual machines run separate full OS instances. -> Option D

4. Quick Check:

   Containers share OS, VMs run full OS [OK]

Hint: Containers share OS; VMs run full OS instances [OK]

Common Mistakes:

• Thinking containers run full OS instances
• Believing VMs share the host OS kernel
• Assuming containers use more disk space

2. Which of the following is the correct command to list running Docker containers?

easy

A. docker ps

B. docker images

C. docker run

D. docker start

Solution

1. Step 1: Identify command to list running containers

   The command docker ps shows all running containers.

2. Step 2: Differentiate from other commands

   docker images lists images, docker run starts a container, and docker start restarts stopped containers.

3. Final Answer:

   docker ps -> Option A

4. Quick Check:

   List running containers = docker ps [OK]

Hint: Use 'docker ps' to see running containers [OK]

Common Mistakes:

• Using 'docker images' to list containers
• Confusing 'docker run' with listing
• Using 'docker start' instead of listing

3. Given the following scenario: You start a Docker container and run uname -a inside it. What will this command output compared to running uname -a on a virtual machine?

medium

A. Both outputs will be identical because containers and VMs have the same kernel.

B. Both outputs will be completely unrelated to the host system.

C. The container's output shows the host kernel, VM shows its own kernel version.

D. The container shows a different kernel version than the host, VM shows host kernel.

Solution

1. Step 1: Understand container kernel sharing

   Containers share the host OS kernel, so uname -a inside a container shows the host kernel version.

2. Step 2: Understand VM kernel isolation

   Virtual machines run their own OS kernel, so uname -a inside a VM shows the VM's kernel version, which can differ from the host.

3. Final Answer:

   The container's output shows the host kernel, VM shows its own kernel version. -> Option C

4. Quick Check:

   Container shares kernel, VM has own kernel [OK]

Hint: Containers share kernel; VMs have separate kernels [OK]

Common Mistakes:

• Assuming containers have separate kernels
• Thinking VM shares host kernel
• Believing outputs are identical always

4. You tried to run a Docker container but got an error saying the container cannot start due to kernel incompatibility. What is the most likely cause?

medium

A. The Docker daemon is not installed.

B. The container image requires a different kernel feature not supported by the host OS.

C. The container image is corrupted and cannot be loaded.

D. The container is trying to run a full OS which is not allowed.

Solution

1. Step 1: Analyze kernel incompatibility error

   Containers rely on the host kernel. If the container needs kernel features missing on the host, it cannot start.

2. Step 2: Rule out other causes

   Docker daemon missing causes different errors; corrupted images cause load errors; containers can run full OS but share kernel.

3. Final Answer:

   The container image requires a different kernel feature not supported by the host OS. -> Option B

4. Quick Check:

   Kernel features mismatch causes container start failure [OK]

Hint: Kernel mismatch blocks container start, check host features [OK]

Common Mistakes:

• Assuming Docker daemon missing causes kernel errors
• Blaming image corruption for kernel issues
• Thinking containers run full OS independently

5. You want to deploy an application that requires full OS isolation for security but also needs to start quickly and use minimal resources. Which approach is best?

hard

A. Use containers with additional security layers like SELinux or AppArmor.

B. Use containers for speed and efficiency but accept less isolation.

C. Use virtual machines for full isolation despite slower startup and higher resource use.

D. Use virtual machines with lightweight OS images to speed up startup.

Solution

1. Step 1: Understand isolation vs speed tradeoff

   Virtual machines provide full OS isolation but are slower and heavier; containers are fast but share kernel.

2. Step 2: Consider enhanced container security

   Adding security layers like SELinux/AppArmor to containers improves isolation while keeping speed and resource benefits.

3. Step 3: Evaluate other options

   Using VMs sacrifices speed; lightweight VMs help but still slower than containers; plain containers lack full isolation.

4. Final Answer:

   Use containers with additional security layers like SELinux or AppArmor. -> Option A

5. Quick Check:

   Secure containers balance isolation and speed [OK]

Hint: Secure containers with extra layers for isolation and speed [OK]

Common Mistakes:

• Choosing VMs ignoring startup speed
• Using plain containers without security layers
• Assuming lightweight VMs match container speed