Docker/Volumes

Basics

Working Directory

First, we can set the working directory when we run a container by using the -w flag:

$ docker  run -w /path/to/dir/ -i -t  ubuntu pwd

This starts a new ubuntu image called pwd with the current working directory (when it starts up) set to /path/to/dir.

Setting Disk Space

We can set the amount of storage for the docker container using --storage-opt flag:

$ docker run -it --storage-opt size=120G fedora /bin/bash

This starts a fedora image with a bash shell, and uses 120 gb for the container.

Mounting Host Folders

Can mount folders on the host machine to drives in the docker machine:

$ docker  run  \
  -v `pwd`:`pwd` \
  -w `pwd` \
  -i -t  ubuntu pwd

This starts a docker container with the name "pwd", and mounts the current working directory (say, /home/someone/docker) to the same path in the docker image /home/someone/docker. It then sets the working directory to that directory.

Making Container Filesystem Read-Only

Read only control can be set on volumes as well: --read-only flag. This flag makes the entire contents of the container's root filesystem read-only, except for volumes mounted with the -v flag. To illustrate:

$ # this will not work, because /canttouchthis is part of the container's root filesystem
$ docker run -t --read-only -v /icanwrite busybox touch /canttouchthis
touch: /canttouchthis: Read-only file system

$ # this will work, because /icanwrite is mounted with -v and is not read-only
$ docker run --read-only -v /icanwrite busybox touch /icanwrite/here

$ # no error, no poblem!

Making Host Directories Read-Only

To mount a host directory on the container's filesystem, you can use the -v flag. To mount a host directory as read-only on the container's filesystem, add :ro to the end of the flag:

$ docker run -v <host path>:/<container path>:ro ...

For example:

$ docker run -v /home/someone/scripts:/scripts:ro -it ubuntu
root@a53d902e433b:/#
root@a53d902e433b:/# touch /scripts/file
touch: cannot touch '/scripts/file': Read-only file system 

Patterns

This section covers some patterns for organizing files so that you can move things in and out from containers, while also keeping in line with the philosophy behind docker containers, which is that they should be stateless.

A data volume is a specially-designated directory within one or more containers that bypasses the Union File System to provide several useful features for persistent or shared data:

• Data volumes can be shared and reused between containers
• Changes to a data volume are made directly
• Changes to a data volume will not be included when you update an image

Docker Volumes tutorial