---
description: Learn the technologies that support storage drivers.
keywords: container, storage, driver, AUFS, btfs, devicemapper,zvfs
title: About images, containers, and storage drivers
redirect_from:
- /en/latest/terms/layer/
- /engine/installation/userguide/storagedriver/
---

To use storage drivers effectively, you must understand how Docker builds and
stores images. Then, you need an understanding of how these images are used by
containers. Finally, you'll need a short introduction to the technologies that
enable both images and container operations.

Understanding how Docker manages the data within your images and containers will
help you understand the best way to design your containers and Dockerize your
applications, and avoid performance problems along the way.

## Images and layers

A Docker image is built up from a series of layers. Each layer represents an
instruction in the image's Dockerfile. Each layer except the very last one is
read-only. Consider the following Dockerfile:

```conf
FROM ubuntu:15.04
COPY . /app
RUN make /app
CMD python /app/app.py
```

This Dockerfile contains four commands, each of which creates a layer.  The
`FROM` statement starts out by creating a layer from the `ubuntu:15.04` image.
The `COPY` command adds some files from your Docker client's current directory.
The `RUN` command builds your application using the `make` command. Finally,
the last layer specifies what command to run within the container.

Each layer is only a set of differences from the layer before it. The layers are
stacked on top of each other. When you create a new container, you add a new
writable layer on top of the underlying layers. This layer is often called the
"container layer". All changes made to the running container, such as writing
new files, modifying existing files, and deleting files, are written to this thin
writable container layer. The diagram below shows a container based on the Ubuntu
15.04 image.

![Docker image layers](images/container-layers.jpg)

A _storage driver_ handles the details about the way these layers interact with
each other. Different storage drivers are available, which have advantages
and disadvantages in different situations.

## Container and layers

The major difference between a container and an image is the top writable layer.
All writes to the container that add new or modify existing data are stored in
this writable layer. When the container is deleted, the writable layer is also
deleted. The underlying image remains unchanged.

Because each container has its own writable container layer, and all changes are
stored in this container layer, multiple containers can share access to the same
underlying image and yet have their own data state. The diagram below shows
multiple containers sharing the same Ubuntu 15.04 image.

![](images/sharing-layers.jpg)

> **Note**: If you need multiple images to have shared access to the exact
> same data, store this data in a Docker volume and mount it into your
> containers.

Docker uses storage drivers to manage the contents of the image layers and the
writable container layer. Each storage driver handles the implementation
differently, but all drivers use stackable image layers and the copy-on-write
(CoW) strategy.

## Container size on disk

To view the approximate size of a running container, you can use the `docker ps -s`
command. Two different columns relate to size.

- `size`: the amount of data (on disk) that is used for the writable layer of
  each container

- `virtual size`: the amount of data used for the read-only image data
  used by the container. Multiple containers may share some or all read-only
  image data. Two containers started from the same image share 100% of the
  read-only data, while two containers with different images which have layers
  in common share those common layers. Therefore, you can't just total the
  virtual sizes. This will over-estimate the total disk usage by a potentially
  non-trivial amount.

The total disk space used by all of the running containers on disk is some
combination of each container's `size` and the `virtual size` values. If
multiple containers have exactly the same `virtual size`, they are likely
started from the same exact image.

This also does not count the following additional ways a container can take up
disk space:

- Disk space used for log files if you use the `json-file` logging driver. This
  can be non-trivial if your container generates a large amount of logging data
  and log rotation is not configured.
- Volumes and bind mounts used by the container.
- Disk space used for the container's configuration files, which are typically
  small.
- Memory written to disk (if swapping is enabled).
- Checkpoints, if you're using the experimental checkpoint/restore feature.

## The copy-on-write (CoW) strategy

Copy-on-write is a strategy of sharing and copying files for maximum efficiency.
If a file or directory exists in a lower layer within the image, and another
layer (including the writable layer) needs read access to it, it just uses the
existing file. The first time another layer needs to modify the file (when
building the image or running the container), the file is copied into that layer
and modified. This minimizes I/O and the size of each of the subsequent layers.
These advantages are explained in more depth below.

### Sharing promotes smaller images

When you use `docker pull` to pull down an image from a repository, or when you
create a container from an image that does not yet exist locally, each layer is
pulled down separately, and stored in Docker's local storage area, which is
usually `/var/lib/docker/` on Linux hosts. You can see these layers being pulled
in this example:

```bash
$ docker pull ubuntu:15.04

15.04: Pulling from library/ubuntu
1ba8ac955b97: Pull complete
f157c4e5ede7: Pull complete
0b7e98f84c4c: Pull complete
a3ed95caeb02: Pull complete
Digest: sha256:5e279a9df07990286cce22e1b0f5b0490629ca6d187698746ae5e28e604a640e
Status: Downloaded newer image for ubuntu:15.04
```

Each of these layers is stored in its own directory inside the Docker host's
local storage area. To examine the layers on the filesystem, list the contents
of `/var/lib/docker/<storage-driver>/layers/`. This example uses `aufs`, which
is the default storage driver:

```bash
$ ls /var/lib/docker/aufs/layers
1d6674ff835b10f76e354806e16b950f91a191d3b471236609ab13a930275e24
5dbb0cbe0148cf447b9464a358c1587be586058d9a4c9ce079320265e2bb94e7
bef7199f2ed8e86fa4ada1309cfad3089e0542fec8894690529e4c04a7ca2d73
ebf814eccfe98f2704660ca1d844e4348db3b5ccc637eb905d4818fbfb00a06a
```

The directory names do not correspond to the layer IDs (this has been true since
Docker 1.10).

Now imagine that you have two different Dockerfiles. You use the first one to
create an image called `acme/my-base-image:1.0`.

```conf
FROM ubuntu:16.10
COPY . /app
```

The second one is based on `acme/my-base-image:1.0`, but has some additional
layers:

```conf
FROM acme/my-base-image:1.0
CMD /app/hello.sh
```

The second image contains all the layers from the first image, plus a new layer
with the `CMD` instruction, and a read-write container layer. Docker already
has all the layers from the first image, so it does not need to pull them again.
The two images will share any layers they have in common.

If you build images from the two Dockerfiles, you can use `docker images` and
`docker history` commands to verify that the cryptographic IDs of the shared
layers are the same.

1.  Make a new directory `cow-test/` and change into it.

2.  Within `cow-test/`, create a new file with the following contents:

    ```bash
    #!/bin/sh
    echo "Hello world"
    ```

    Save the file, and make it executable:

    ```bash
    chmod +x hello.sh
    ```

3.  Copy the contents of the first Dockerfile above into a new file called
    `Dockerfile.base`.

4.  Copy the contents of the second Dockerfile above into a new file called
    `Dockerfile`.

5.  Within the `cow-test/` directory, build the first image.

    ```bash
    $ docker build -t acme/my-base-image:1.0 -f Dockerfile.base .

    Sending build context to Docker daemon  4.096kB
    Step 1/2 : FROM ubuntu:16.10
     ---> 31005225a745
    Step 2/2 : COPY . /app
     ---> Using cache
     ---> bd09118bcef6
    Successfully built bd09118bcef6
    Successfully tagged acme/my-base-image:1.0
    ```

6.  Build the second image.

    ```bash
    $ docker build -t acme/my-final-image:1.0 -f Dockerfile .

    Sending build context to Docker daemon  4.096kB
    Step 1/2 : FROM acme/my-base-image:1.0
     ---> bd09118bcef6
    Step 2/2 : CMD /app/hello.sh
     ---> Running in a07b694759ba
     ---> dbf995fc07ff
    Removing intermediate container a07b694759ba
    Successfully built dbf995fc07ff
    Successfully tagged acme/my-final-image:1.0
    ```

7.  Check out the sizes of the images:

    ```bash
    $ docker images

    REPOSITORY                                            TAG                          IMAGE ID            CREATED             SIZE
    acme/my-final-image                                   1.0                          dbf995fc07ff        58 seconds ago      103MB
    acme/my-base-image                                    1.0                          bd09118bcef6        3 minutes ago       103MB
    ```

8.  Check out the layers that comprise each image:

    ```bash
    $ docker history bd09118bcef6
    IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
    bd09118bcef6        4 minutes ago       /bin/sh -c #(nop) COPY dir:35a7eb158c1504e...   100B                
    31005225a745        3 months ago        /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B                  
    <missing>           3 months ago        /bin/sh -c mkdir -p /run/systemd && echo '...   7B                  
    <missing>           3 months ago        /bin/sh -c sed -i 's/^#\s*\(deb.*universe\...   2.78kB              
    <missing>           3 months ago        /bin/sh -c rm -rf /var/lib/apt/lists/*          0B                  
    <missing>           3 months ago        /bin/sh -c set -xe   && echo '#!/bin/sh' >...   745B                
    <missing>           3 months ago        /bin/sh -c #(nop) ADD file:eef57983bd66e3a...   103MB      
    ```

    ```bash
    $ docker history dbf995fc07ff

    IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
    dbf995fc07ff        3 minutes ago       /bin/sh -c #(nop)  CMD ["/bin/sh" "-c" "/a...   0B                  
    bd09118bcef6        5 minutes ago       /bin/sh -c #(nop) COPY dir:35a7eb158c1504e...   100B                
    31005225a745        3 months ago        /bin/sh -c #(nop)  CMD ["/bin/bash"]            0B                  
    <missing>           3 months ago        /bin/sh -c mkdir -p /run/systemd && echo '...   7B                  
    <missing>           3 months ago        /bin/sh -c sed -i 's/^#\s*\(deb.*universe\...   2.78kB              
    <missing>           3 months ago        /bin/sh -c rm -rf /var/lib/apt/lists/*          0B                  
    <missing>           3 months ago        /bin/sh -c set -xe   && echo '#!/bin/sh' >...   745B                
    <missing>           3 months ago        /bin/sh -c #(nop) ADD file:eef57983bd66e3a...   103MB  
    ```

    Notice that all the layers are identical except the top layer of the second
    image. All the other layers are shared between the two images, and are only
    stored once in `/var/lib/docker/`. The new layer actually doesn't take any
    room at all, because it is not changing any files, but only running a command.

    > **Note**: The `<missing>` lines in the `docker history` output indicate
    > that those layers were built on another system and are not available
    > locally. This can be ignored.

### Copying makes containers efficient

When you start a container, a thin writable container layer is added on top of
the other layers. Any changes the container makes to the filesystem are stored
here. Any files the container does not change do not get copied to this writable
layer. This means that the writable layer is as small as possible.

When an existing file in a container is modified, the storage driver performs a
copy-on-write operation. The specifics steps involved depend on the specific
storage driver. For the default `aufs` driver and the `overlay` and `overlay2`
drivers, the copy-on-write operation follows this rough sequence:

*  Search through the image layers for the file to update. The process starts
   at the newest layer and works down to the base layer one layer at a time.
   When results are found, they are added to a cache to speed future operations.

*  Perform a `copy_up` operation on the first copy of the file that is found, to
   copy the file to the container's writable layer.

*  Any modifications are made to this copy of the file, and the container cannot
   see the read-only copy of the file that exists in the lower layer.

Btrfs, ZFS, and other drivers handle the copy-on-write differently. You can
read more about the methods of these drivers later in their detailed
descriptions.

Containers that write a lot of data will consume more space than containers
that do not. This is because most write operations consume new space in the
container's thin writable top layer.

> **Note**: for write-heavy applications, you should not store the data in
> the container. Instead, use Docker volumes, which are independent of the
> running container and are designed to be efficient for I/O. In addition,
> volumes can be shared among containers and do not increase the size of your
> container's writable layer.

A `copy_up` operation can incur a noticeable performance overhead. This overhead
is different depending on which storage driver is in use. Large files,
lots of layers, and deep directory trees can make the impact more noticeable.
This is mitigated by the fact that each `copy_up` operation only occurs the first
time a given file is modified.

To verify the way that copy-on-write works, the following procedures spins up 5
containers based on the `acme/my-final-image:1.0` image we built earlier and
examines how much room they take up.

> **Note**: This procedure won't work on Docker for Mac or Docker for Windows.

1.  From a terminal on your Docker host, run the following `docker run` commands.
    The strings at the end are the IDs of each container.

    ```bash
    $ docker run -dit --name my_container_1 acme/my-final-image:1.0 bash \
      && docker run -dit --name my_container_2 acme/my-final-image:1.0 bash \
      && docker run -dit --name my_container_3 acme/my-final-image:1.0 bash \
      && docker run -dit --name my_container_4 acme/my-final-image:1.0 bash \
      && docker run -dit --name my_container_5 acme/my-final-image:1.0 bash

      c36785c423ec7e0422b2af7364a7ba4da6146cbba7981a0951fcc3fa0430c409
      dcad7101795e4206e637d9358a818e5c32e13b349e62b00bf05cd5a4343ea513
      1e7264576d78a3134fbaf7829bc24b1d96017cf2bc046b7cd8b08b5775c33d0c
      38fa94212a419a082e6a6b87a8e2ec4a44dd327d7069b85892a707e3fc818544
      1a174fc216cccf18ec7d4fe14e008e30130b11ede0f0f94a87982e310cf2e765
    ```


2.  Run the `docker ps` command to verify the 5 containers are running.

    ```bash
    CONTAINER ID        IMAGE                     COMMAND                  CREATED              STATUS              PORTS               NAMES
    1a174fc216cc        acme/my-final-image:1.0   "bash"                   About a minute ago   Up About a minute                       my_container_5
    38fa94212a41        acme/my-final-image:1.0   "bash"                   About a minute ago   Up About a minute                       my_container_4
    1e7264576d78        acme/my-final-image:1.0   "bash"                   About a minute ago   Up About a minute                       my_container_3
    dcad7101795e        acme/my-final-image:1.0   "bash"                   About a minute ago   Up About a minute                       my_container_2
    c36785c423ec        acme/my-final-image:1.0   "bash"                   About a minute ago   Up About a minute                       my_container_1
    ```


3.  List the contents of the local storage area.

    ```bash
    $ sudo ls /var/lib/docker/containers

    1a174fc216cccf18ec7d4fe14e008e30130b11ede0f0f94a87982e310cf2e765
    1e7264576d78a3134fbaf7829bc24b1d96017cf2bc046b7cd8b08b5775c33d0c
    38fa94212a419a082e6a6b87a8e2ec4a44dd327d7069b85892a707e3fc818544
    c36785c423ec7e0422b2af7364a7ba4da6146cbba7981a0951fcc3fa0430c409
    dcad7101795e4206e637d9358a818e5c32e13b349e62b00bf05cd5a4343ea513
    ```

4.  Now check out their sizes:

    ```bash
    $ sudo du -sh /var/lib/docker/containers/*

    32K  /var/lib/docker/containers/1a174fc216cccf18ec7d4fe14e008e30130b11ede0f0f94a87982e310cf2e765
    32K  /var/lib/docker/containers/1e7264576d78a3134fbaf7829bc24b1d96017cf2bc046b7cd8b08b5775c33d0c
    32K  /var/lib/docker/containers/38fa94212a419a082e6a6b87a8e2ec4a44dd327d7069b85892a707e3fc818544
    32K  /var/lib/docker/containers/c36785c423ec7e0422b2af7364a7ba4da6146cbba7981a0951fcc3fa0430c409
    32K  /var/lib/docker/containers/dcad7101795e4206e637d9358a818e5c32e13b349e62b00bf05cd5a4343ea513
    ```

    Each of these containers only takes up 32k of space on the filesystem.

Not only does copy-on-write save space, but it also reduces start-up time.
When you start a container (or multiple containers from the same image), Docker
only needs to create the thin writable container layer.

If Docker had to make an entire copy of the underlying image stack each time it
started a new container, container start times and disk space used would be
significantly increased. This would be similar to the way that virtual machines
work, with one or more virtual disks per virtual machine.

## Data volumes and the storage driver

When a container is deleted, any data written to the container that is not
stored in a *data volume* is deleted along with the container.

A data volume is a directory or file in the Docker host's filesystem that is
mounted directly into a container. Data volumes are not controlled by the
storage driver. Reads and writes to data volumes bypass the storage driver and
operate at native host speeds. You can mount any number of data volumes into a
container. Multiple containers can also share one or more data volumes.

The diagram below shows a single Docker host running two containers. Each
container exists inside of its own address space within the Docker host's local
storage area (`/var/lib/docker/...`). There is also a single shared data volume
located at `/data` on the Docker host. This is mounted directly into both
containers.

![](images/shared-volume.jpg)

Data volumes reside outside of the local storage area on the Docker host,
further reinforcing their independence from the storage driver's control. When
a container is deleted, any data stored in data volumes persists on the Docker
host.

For detailed information about data volumes, see
[Managing data in containers](/engine/tutorials/dockervolumes/).

## Related information

* [Select a storage driver](selectadriver.md)
* [AUFS storage driver in practice](aufs-driver.md)
* [Btrfs storage driver in practice](btrfs-driver.md)
* [Device Mapper storage driver in practice](device-mapper-driver.md)
