tmpfs mounts

Volumes and bind mounts let you share files between the host machine and container so that you can persist data even after the container is stopped.

If you're running Docker on Linux, you have a third option: tmpfs mounts. When you create a container with a tmpfs mount, the container can create files outside the container's writable layer.

As opposed to volumes and bind mounts, a tmpfs mount is temporary, and only persisted in the host memory. When the container stops, the tmpfs mount is removed, and files written there won't be persisted.

tmpfs mounts are best used for cases when you do not want the data to persist either on the host machine or within the container. This may be for security reasons or to protect the performance of the container when your application needs to write a large volume of non-persistent state data.

Important

tmpfs mounts in Docker map directly to tmpfs in the Linux kernel. As such, the temporary data may be written to a swap file, and thereby persisted to the filesystem.

Mounting over existing data

If you create a tmpfs mount into a directory in the container in which files or directories exist, the pre-existing files are obscured by the mount. This is similar to if you were to save files into /mnt on a Linux host, and then mounted a USB drive into /mnt. The contents of /mnt would be obscured by the contents of the USB drive until the USB drive was unmounted.

With containers, there's no straightforward way of removing a mount to reveal the obscured files again. Your best option is to recreate the container without the mount.

Limitations of tmpfs mounts

• Unlike volumes and bind mounts, you can't share tmpfs mounts between containers.
• This functionality is only available if you're running Docker on Linux.
• Setting permissions on tmpfs may cause them to reset after container restart. In some cases setting the uid/gid can serve as a workaround.

Syntax

To mount a tmpfs with the docker run command, you can use either the --mount or --tmpfs flag.

$ docker run --mount type=tmpfs,dst=<mount-path>
$ docker run --tmpfs <mount-path>

In general, --mount is preferred. The main difference is that the --mount flag is more explicit and supports all the available options.

The --tmpfs flag cannot be used with swarm services. You must use --mount.

Options for --mount

The --mount flag consists of multiple key-value pairs, separated by commas and each consisting of a <key>=<value> tuple. The order of the keys isn't significant.

$ docker run --mount type=tmpfs,dst=<mount-path>[,<key>=<value>...]

Valid options for --mount type=tmpfs include:

$ docker run --mount type=tmpfs,dst=/app,tmpfs-size=21474836480,tmpfs-mode=1770

Options for --tmpfs

The --tmpfs flag does not let you specify any options.

Use a tmpfs mount in a container

To use a tmpfs mount in a container, use the --tmpfs flag, or use the --mount flag with type=tmpfs and destination options. There is no source for tmpfs mounts. The following example creates a tmpfs mount at /app in a Nginx container. The first example uses the --mount flag and the second uses the --tmpfs flag.

$ docker run -d \
  -it \
  --name tmptest \
  --mount type=tmpfs,destination=/app \
  nginx:latest

$ docker run -d \
  -it \
  --name tmptest \
  --tmpfs /app \
  nginx:latest

Verify that the mount is a tmpfs mount by looking in the Mounts section of the docker inspect output:

$ docker inspect tmptest --format '{{ json .Mounts }}'
[{"Type":"tmpfs","Source":"","Destination":"/app","Mode":"","RW":true,"Propagation":""}]

Stop and remove the container:

$ docker stop tmptest
$ docker rm tmptest

Next steps

• Learn about volumes
• Learn about bind mounts
• Learn about storage drivers