DevOps Classroomnotes 22/Nov/2022

Problem with Persistence in Containers

• Each Docker container gets an thin read-write layer, where the files generated/modified by application are present.
• Docker container gets a filesystem which is collection of image layers plus read-write layer mounted on each other but it looks like one filesystem.
• When container tries to edit existing files in image layers , this will not impact image layers as image layers are read-only.
• Internally docker storage system copies the files from read-only image layers to thin read-write layer before modification. This strategy is called as Copy-on-write
  
• Refer Here for storage drivers.
• Docker acheives this functionality with the help of storage drivers.
• The lifetime of Read-Write layer is equal to lifetime of container i.e. once the container is deleted all the contents of writable layer are also deleted.
• Consider we have a database container which is used by other app containers. The data is written into db container from app containers i.e. all of this is present in Writable layer . Now if we delete db container all the data is lost.
• To solve this problem docker has introduced the concept of Volumes.

Docker Volumes

• Refer Here for the blog on Docker volumes
• Refer Here for the official docs
• Docker volumes purpose is to persist the data even after container is deleted.
• Docker Volume types
  • bind mount
  • Volume
  • tmpfs
• I want to preserve what ever is present in tmp directory of alpine container.
• Bind mount: Mount existing folder from docker host to any folder in container.
  • Create a folder
  • mount it to the container
  • Create some files
    
  • Delete the container and cross check the files
  • Now create a new container and mount the files to the same path
    
  • We can also share the same folder from docker host to multiple containers.
  • The same thing can be acheived with –mount
    docker container run -d --mount "type=bind,source=/tmp/cont-temp,target=/tmp" --name bind6 alpine sleep 1d
    
• Volume Mounts: These are storage spaces generally from docker host which is managed by docker. docker has a sub command docker volume
  
  
• Lets mount the tools folder in alpine container with my-vol
  
• Delete the container
  
• To remove the volume execute docker volume rm
• Experiment: Let me create a mysql container and postgres container
  • Lets see the list of volumes
    
  • mysql docker run --name some-mysql -e MYSQL_ROOT_PASSWORD=my-secret-pw -d mysql and list volumes
    
  • When we created mysql one volume got created.
  • When we create postgres as well a volume got created automatically
    
  • This is happening because of VOLUME instruction in Dockerfile.