Containers vs virtual machines in Docker - Visual Side-by-Side Comparison

Process Flow - Containers vs virtual machines

Start

↓

User requests app

↓

Virtual Machine

↓

Guest OS boots

↓

App runs inside VM

↓

Container

↓

Container Engine

↓

App runs in container

↓

Host OS

Shows how user requests run either inside a full virtual machine with its own OS or inside a container sharing the host OS.

Execution Sample

Docker

docker run --name mycontainer nginx
# Starts an nginx container

# Virtual machine example (conceptual):
# VM boots full OS then runs nginx

Starts an nginx app inside a container quickly without booting a full OS, unlike a VM.

Process Table

Step	Action	Environment	Resource Used	Result
1	User runs 'docker run nginx'	Host OS	Docker Engine	Container created and started
2	Container shares Host OS kernel	Container	Host OS Kernel	No separate OS boot needed
3	Nginx process starts inside container	Container	Isolated filesystem and network	Nginx ready to serve
4	User accesses nginx	Host OS	Network forwarding	Nginx serves web page
5	User starts VM with nginx	Host OS	Hypervisor	VM boots full guest OS
6	Guest OS boots	VM	CPU, RAM	Full OS running
7	Nginx starts inside VM	VM	Guest OS resources	Nginx ready to serve
8	User accesses nginx in VM	Host OS	Network forwarding	Nginx serves web page
9	Stop container	Host OS	Docker Engine	Container stopped quickly
10	Stop VM	Host OS	Hypervisor	VM shuts down slowly

💡 Execution stops after container and VM lifecycle steps complete

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	After Step 7	Final
Container State	Not running	Created	Running	N/A	N/A	Stopped
VM State	Not running	N/A	N/A	Starting	Running	Stopped
Nginx Status	Not running	Not running	Running	Not running	Running	Not running

Key Moments - 3 Insights

Why does the container start faster than the virtual machine?

Because the container shares the host OS kernel and does not boot a full OS, as shown in execution_table steps 2 and 6.

Does a container have its own operating system?

No, containers share the host OS kernel but have isolated filesystems and processes, unlike VMs which have full guest OSes (see steps 2 and 6).

Why does stopping a VM take longer than stopping a container?

Stopping a VM requires shutting down the full guest OS, while stopping a container just stops the isolated process (see steps 9 and 10).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the container start running nginx?

A Step 3

B Step 2

C Step 5

D Step 7

💡 Hint

Check the 'Nginx process starts inside container' row in execution_table

At which step does the VM finish booting its guest OS?

A Step 4

B Step 6

C Step 7

D Step 8

💡 Hint

Look for 'Guest OS boots' in execution_table

If the container shared its own OS instead of the host OS, how would the start time change?

A It would start faster

B No change

C It would start slower

D It would not start at all

💡 Hint

Compare container start (step 1-3) with VM boot (step 5-7) in execution_table

Concept Snapshot

Containers run apps sharing the host OS kernel, starting quickly with isolated filesystems.
Virtual machines boot full guest OSes, using more resources and starting slower.
Containers are lightweight and fast; VMs provide full OS isolation.
Use containers for fast deployment, VMs for full OS environments.

Full Transcript

This visual execution compares containers and virtual machines. When a user runs a container, Docker creates an isolated environment sharing the host OS kernel, so it starts quickly without booting a full OS. The nginx app runs inside this container with isolated filesystem and network. In contrast, starting a virtual machine requires booting a full guest operating system inside a hypervisor, which takes more time and resources. The VM runs nginx inside its own OS. Stopping containers is faster because it only stops the isolated process, while stopping VMs involves shutting down the entire guest OS. This shows containers are lightweight and fast, while VMs provide full OS isolation but are heavier.

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