DevOps Classroom notes 13/Dec/2025

Docker image layers and container

• Every container gets an virtual disk which is collection of
  • readonly layers (reusable across containers using same image)
  • writable layer (unique for every container)
    
• For this to workout docker relies on storage driver, as of now overlay driver does this.
• Any changes from files in readonly layers will be copied into writable layer and then modified.
• When we delete the container the writable layer also gets deleted i.e. data loss happens.

Solution: Docker volumes

• figure out the folder where you are storing the data i.e. required (should not be lost)
• For convienience lets assume the folder is /data
• We need to ensure this folder lifetime has no dependency on container lifetime.
• Docker volumes help in creating storages which can have lifetime independent of containers.
• docker volumes can be created in two ways
  • Mount some folder on docker host into docker container
  • Ask docker to manage this folder

Option 1: Mounting some folder on host into container

• Create a new folder mkdir /root/data
• now lets create an alpine container

docker run -d --name alp1 -v /root/data:/data  alpine sleep 1d

Option 2: Create a docker volume and then mount

• Create a new volume

docker volume create mydata

• now lets create an alpine container

docker run -d --name alp1 -v mydata:/data  alpine sleep 1d

• docker volumes with extensions installed can store volumes in aws, azure, or even external locations
• Docker supports VOLUME instruction in Dockerfile. This instruction automatically creates an anonymous volume if the user doesn’t create explicitly.

Lets create a mysql container

• command

# create a volume
docker volume create nopdb
# create a mysql contianer
docker run -d \
   -p 33061:3306  \
   --name nopdb \
   -v nopdb:/var/lib/mysql \
   -e MYSQL_ROOT_PASSWORD=admin123 \
   -e MYSQL_DATABASE=nop \
   -e MYSQL_USER=nop \
   -e MYSQL_PASSWORD=nop123 \
   mysql:9

• Watch classroom recording for next steps
• Exercise: Repeat the same with postgres db and mongodb in a docker container.

Next Steps

• Networking
• optimizations
• Microservice
• docker compose