Docker Volumes

I think it’s hard to overstate how revolutionary Docker has been for the tech industry. A lot of use cases that would have been served by a VM are typically replaced by Docker Containers.

Good I say - having spent enough time with my elbows deep inside a VM (so to speak), I appreciate the small weight of containers. Unless you’re working strictly in the on-premises Microsoft/Windows Server stack, it’s the defacto standard for deploying applications.

Getting started is pretty easy, but diving deeper has many footguns to stumble upon. When you’re starting out, I consider data persistence one of those cases. Docker provides options for storing data - I’m going to focus on Docker Volumes.

Docker Docs - Volumes

In the link above, I’m focusing on the section Choose the -v or –mount flag. I’ve got a couple of commands that show how using -v or --mount can result in either a bind-mount or a named volume:

Bind-mount

docker run --name pg \
    -d \
    -p 5432:5432 \
    -v /Users/ilanham/pgdata:/var/lib/postgresql/data \
    -e POSTGRES_PASSWORD=<secret> \
    postgres

The trick above is after the -v flag, where I specify a physical file path to the left of the colon. If the path doesn’t exist on the filesystem before I try to create this container, Docker will create it.

docker run --name pg \
    -d \
    -p 5432:5432 \
    --mount type=volume,source=sample,target=/var/lib/postgresql/data \
    -e POSTGRES_PASSWORD=<secret> \
    postgres

For the named volume, I’m using the --mount syntax, but I can just as easily use -v

docker run --name pg \
    -d \
    -p 5432:5432 \
    -v quick_test:/var/lib/postgresql/data \
    -e POSTGRES_PASSWORD=<secret> \
    postgres

Where are my named volumes?

• Linux: /var/lib/docker/volumes
• MacOS: ~/Library/Containers/com.docker.docker/Data/vms/#¹
• Windows: \\wsl$\docker-desktop-data\data\docker\volumes\<vol_name>²