docker-volumes

Commands

Create - create a new volume

• additional configuration is required for created volumes to be used in docker compose
  ls - list the available volumes names and its corresponding drivers
  rm - remove a volume that is not used by a container
  Prune - remove volumes that are not used by at least 1 container
  Find what container is using what volume

docker ps -a --filter volume=VOLUME_NAME_OR_MOUNT_POINT

Overview

Docker Volumes are stored in /var/lib/docker/volumes/[named or random[/_data and all the files are owned by root regardless of user ID. For every VOLUME in dockerfile a volume will be created and the user will have an option to bind mount or named volume; if none are chosen, a default named volume with random letters will be created automatically by docker.

Types of Volumes

Bind

docker run -v /path/to/folder:/folder

• the files are managed by the system and retain the permissions

Folder Contents

⚠ If a folder does not exist → Docker will create a folder under root user
If a folder already exists or it contains content → The prescence/content of that folder will override existing files already in the container (not the case with named volumes)

• :rw/ro makes a container read-only

Named Volume

docker run -v myvol:/app

• the files and permissions are managed by Docker, useful if the container itself do not respect PUID/PGID for file permissions, also good for databases

tmpfs

services:
  app:
    tmpfs:
	  - /unused

Using tmpfs prevents Docker from creating a anonymous volume when spinning up a container with VOLUME in the Dockerfile.

Filesystem

Storage

docker ps -s

• gets the size/virtual size of docker container
• size - container size
• virtual size - size of image/shared
  Docker uses a union filesystem (similar to MergerFS) that combine the data into all layers. The files within containers are stored in /var/lib/docker/overlay2
  
• Writable Layer - all the writes of a Docker container goes to this layer and will be deleted when container is gone
• upper layer - the writable layer or container layer
• lower layer - the read only layer or image layer
• merged - the layer consist of merged files from upper and lower dirs
  This storage setup have different file operations
• Reading files that exists in both upper/lower dir - Docker will read the dir that is layered on top
• Overwriting file - rather than overwriting the file directly which is read only, it will create a new file
• Deleting file - since Docker cannot delete on the read-only layer, a whiteout file is written on upper layer and thus increasing image size