🚀 Day 13: Mastering Docker Volumes | Complete Guide

Welcome to this complete guide on Docker Volumes! In this video, we'll go from basics to advanced topics, ensuring you're fully equipped to manage persistent data in Docker. Let's dive in! 🐳💡

📌 What Are Docker Volumes?

Docker volumes are persistent storage mechanisms used by containers to store data outside of the container’s writable layer.

• Containers are ephemeral – when removed, their data is lost.

• Volumes ensure your data persists, even after container deletion. like: Logs, generated files, user upload 🧠 Think of volumes as hard drives attached to your containers.

Why Use Docker Volumes?

Data Persistence: Retain data even after the container is stopped or deleted. Data Sharing: Share data between multiple containers. Ease of Backup: Volumes can be easily backed up and restored. Performance: Volumes provide better I/O performance than bind mounts.

🛠️ Types of Docker Storage

There are three main storage options in Docker:

1. Volumes (Managed by Docker)

2. Bind mounts (link to host machine directories)

3. tmpfs mounts (stored in memory)

💡 Volumes are the most portable, secure, and recommended option.

📌Docker Container Without volume

But before understanding above volumes we will see first losses of data when we go with without volumes. Here we will understand how Docker manages read-write layers, data persistence, and what happens when volumes are not used. We'll simulate and observe:

• Layer creation

• File system overlays

• What gets persisted, what gets lost

• Tracking custom changes

• Container layer internals with real inspection 🕵️‍♂️

🧱 Conceptual Foundation

🧪 Step-by-Step Hands-On Practice (Without Docker Volumes)

🔹 Step 1: Pull and Inspect Image

docker pull ubuntu:latest
docker image history --no-trunc ubuntu:latest
docker image history ubuntu:latest
# We will see deep dive of image layers in image video series.
dive ubuntu:latest  
# Visual tool for layer inspection (install if needed)
# https://github.com/wagoodman/dive

🔹 Step 2: Run Container Without Volume

docker run -itd --name=novolume ubuntu /bin/bash

Inside the container:

docker ps
docker exec -it novolume /bin/bash
echo "Welcome to DevOps in Action!" > ibrar.txt
cat ibrar.txt
exit

Understand The Docker Filesystem

Docker containers run the software stack defined within an Docker image. Images are made of a set of read-only layers that work on a filesystem called the Union Filesystem. When we start a new container, Docker adds a read-write layer on top of the image layers enabling the container to run as if it’s on a standard Linux filesystem.

So, any file change inside the container creates a working copy in the read-write layer. However, when the container is stopped or deleted, that read-write layer is lost:

🔹 Step 3: Verify Container and Find File Location and Deference

# Get layered details
docker inspect novolume | jq '.[0].GraphDriver'
# ID: Unique identifier for the overlay2 layer used by the container.
# LowerDir: Read-only base layers stacked below the container's writable layer.
# MergedDir: The unified view (read/write) of all layers mounted and used by the container.
# UpperDir: The top writable layer where changes made by the container are stored.
# WorkDir: Temporary working directory used internally by overlay2 for layer operations.
# Name: Name of the storage driver in use (in this case, overlay2).

# Check UpperDir
docker inspect novolume --format '{{ .GraphDriver.Data.UpperDir }}'

# Check differences
docker diff novolume
or
UpperDir=$(docker inspect novolume --format '{{ .GraphDriver.Data.UpperDir }}')
sudo ls $UpperDir
sudo cat $UpperDir/ibrar.txt
sudo stat $UpperDir/ibrar.txt

👉 You’ll see ibrar.txt in the RW layer.

🔹 Step 4: Container Restart – Persistence Check

docker restart novolume
docker exec -it novolume /bin/bash
cat ibrar.txt
exit

✅ File still exists — because container is still active.

🔻 Step 5: Container Deletion = Data Loss!

# Remove container
docker rm -f novolume
# Check Data loss:
UpperDir=$(docker inspect novolume --format '{{ .GraphDriver.Data.UpperDir }}')
sudo ls $UpperDir/ibrar.txt # ❌ File is gone

# Recreate again to recover data loss
docker run -itd --name=novolume ubuntu /bin/bash
# Again Check Data loss:
sudo ls $UpperDir/ibrar.txt # ❌ File is gone no recovery possible.

🧪What Happens When You Delete a Docker Container?

Do practice of below containers from your end and see data losses.

# MySQL
docker run -d --name mysql -e MYSQL_ROOT_PASSWORD=rootpassword -e MYSQL_DATABASE=mydb -e MYSQL_USER=myuser -e MYSQL_PASSWORD=mypassword -p 3306:3306 mysql:8.0
# Jenkins
docker run -d --name jenkins -p 8080:8080 -p 50000:50000 jenkins/jenkins:lts
# Grafana
docker run -d --name=grafana -p 3000:3000 grafana/grafana

🛡️ Step 6: How to Avoid Data Loss

To prevent data loss, you can use:

• Docker Volumes: Persistent storage managed by Docker.(Reccomended)

• Bind Mounts: Map a host directory to a container path.(Good for Dev and Test)

• Copy Data Out: Use docker cp to extract files before removing containers(Not a best practices.

🎯 Final Thoughts

Always use Docker volumes in production to avoid data loss and for better performance.