github.com/hustcat/docker@v1.3.3-0.20160314103604-901c67a8eeab/docs/userguide/storagedriver/aufs-driver.md (about)

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     3  title = "AUFS storage driver in practice"
     4  description = "Learn how to optimize your use of AUFS driver."
     5  keywords = ["container, storage, driver, AUFS "]
     6  [menu.main]
     7  parent = "engine_driver"
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     9  <![end-metadata]-->
    10  
    11  # Docker and AUFS in practice
    12  
    13  AUFS was the first storage driver in use with Docker. As a result, it has a
    14  long and close history with Docker, is very stable, has a lot of real-world
    15  deployments, and has strong community support. AUFS has several features that
    16  make it a good choice for Docker. These features enable:
    17  
    18  - Fast container startup times.
    19  - Efficient use of storage.
    20  - Efficient use of memory.
    21  
    22  Despite its capabilities and long history with Docker, some Linux distributions
    23   do not support AUFS. This is usually because AUFS is not included in the
    24  mainline (upstream) Linux kernel.
    25  
    26  The following sections examine some AUFS features and how they relate to
    27  Docker.
    28  
    29  ## Image layering and sharing with AUFS
    30  
    31  AUFS is a *unification filesystem*. This means that it takes multiple
    32  directories on a single Linux host, stacks them on top of each other, and
    33  provides a single unified view. To achieve this, AUFS uses a *union mount*.
    34  
    35  AUFS stacks multiple directories and exposes them as a unified view through a
    36  single mount point. All of the directories in the stack, as well as the union
    37  mount point, must all exist on the same Linux host. AUFS refers to each
    38  directory that it stacks as a *branch*.
    39  
    40  Within Docker, AUFS union mounts enable image layering. The AUFS storage driver
    41   implements Docker image layers using this union mount system. AUFS branches
    42  correspond to Docker image layers. The diagram below shows a Docker container
    43  based on the `ubuntu:latest` image.
    44  
    45  ![](images/aufs_layers.jpg)
    46  
    47  This diagram shows that each image layer, and the container layer, is
    48  represented in the Docker hosts filesystem as a directory under
    49  `/var/lib/docker/`. The union mount point provides the unified view of all
    50  layers. As of Docker 1.10, image layer IDs do not correspond to the names of
    51  the directories that contain their data.
    52  
    53  AUFS also supports the copy-on-write technology (CoW). Not all storage drivers
    54  do.
    55  
    56  ## Container reads and writes with AUFS
    57  
    58  Docker leverages AUFS CoW technology to enable image sharing and minimize the
    59  use of disk space. AUFS works at the file level. This means that all AUFS CoW
    60  operations copy entire files - even if only a small part of the file is being
    61  modified. This behavior can have a noticeable impact on container performance,
    62   especially if the files being copied are large, below a lot of image layers,
    63  or the CoW operation must search a deep directory tree.
    64  
    65  Consider, for example, an application running in a container needs to add a
    66  single new value to a large key-value store (file). If this is the first time
    67  the file is modified, it does not yet exist in the container's top writable
    68  layer. So, the CoW must *copy up* the file from the underlying image. The AUFS
    69  storage driver searches each image layer for the file. The search order is from
    70   top to bottom. When it is found, the entire file is *copied up* to the
    71  container's top writable layer. From there, it can be opened and modified.
    72  
    73  Larger files obviously take longer to *copy up* than smaller files, and files
    74  that exist in lower image layers take longer to locate than those in higher
    75  layers. However, a *copy up* operation only occurs once per file on any given
    76  container. Subsequent reads and writes happen against the file's copy already
    77  *copied-up* to the container's top layer.
    78  
    79  ## Deleting files with the AUFS storage driver
    80  
    81  The AUFS storage driver deletes a file from a container by placing a *whiteout
    82  file* in the container's top layer. The whiteout file effectively obscures the
    83  existence of the file in the read-only image layers below. The simplified
    84  diagram below shows a container based on an image with three image layers.
    85  
    86  ![](images/aufs_delete.jpg)
    87  
    88  The `file3` was deleted from the container. So, the AUFS storage driver  placed
    89  a whiteout file in the container's top layer. This whiteout file effectively
    90  "deletes" `file3` from the container by obscuring any of the original file's
    91  existence in the image's read-only layers. This works the same no matter which
    92  of the image's read-only layers the file exists in.
    93  
    94  ## Configure Docker with AUFS
    95  
    96  You can only use the AUFS storage driver on Linux systems with AUFS installed.
    97  Use the following command to determine if your system supports AUFS.
    98  
    99      $ grep aufs /proc/filesystems
   100      nodev   aufs
   101  
   102  This output indicates the system supports AUFS. Once you've verified your
   103  system supports AUFS, you can must instruct the Docker daemon to use it. You do
   104  this from the command line with the `docker daemon` command:
   105  
   106      $ sudo docker daemon --storage-driver=aufs &
   107  
   108  
   109  Alternatively, you can edit the Docker config file and add the
   110  `--storage-driver=aufs` option to the `DOCKER_OPTS` line.
   111  
   112      # Use DOCKER_OPTS to modify the daemon startup options.
   113      DOCKER_OPTS="--storage-driver=aufs"
   114  
   115  Once your daemon is running, verify the storage driver with the `docker info`
   116  command.
   117  
   118      $ sudo docker info
   119      Containers: 1
   120      Images: 4
   121      Storage Driver: aufs
   122       Root Dir: /var/lib/docker/aufs
   123       Backing Filesystem: extfs
   124       Dirs: 6
   125       Dirperm1 Supported: false
   126      Execution Driver: native-0.2
   127      ...output truncated...
   128  
   129  The output above shows that the Docker daemon is running the AUFS storage
   130  driver on top of an existing `ext4` backing filesystem.
   131  
   132  ## Local storage and AUFS
   133  
   134  As the `docker daemon` runs with the AUFS driver, the driver stores images and
   135  containers within the Docker host's local storage area under
   136  `/var/lib/docker/aufs/`.
   137  
   138  ### Images
   139  
   140  Image layers and their contents are stored under
   141  `/var/lib/docker/aufs/diff/`. With Docker 1.10 and higher, image layer IDs do
   142  not correspond to directory names.
   143  
   144  The `/var/lib/docker/aufs/layers/` directory contains metadata about how image
   145  layers are stacked. This directory contains one file for every image or
   146  container layer on the Docker host (though file names no longer match image
   147  layer IDs). Inside each file are the names of the directories that exist below
   148  it in the stack
   149  
   150  The command below shows the contents of a metadata file in
   151  `/var/lib/docker/aufs/layers/` that lists the three directories that are
   152  stacked below it in the union mount. Remember, these directory names do no map
   153  to image layer IDs with Docker 1.10 and higher.
   154  
   155      $ cat /var/lib/docker/aufs/layers/91e54dfb11794fad694460162bf0cb0a4fa710cfa3f60979c177d920813e267c
   156      d74508fb6632491cea586a1fd7d748dfc5274cd6fdfedee309ecdcbc2bf5cb82
   157      c22013c8472965aa5b62559f2b540cd440716ef149756e7b958a1b2aba421e87
   158      d3a1f33e8a5a513092f01bb7eb1c2abf4d711e5105390a3fe1ae2248cfde1391
   159  
   160  The base layer in an image has no image layers below it, so its file is empty.
   161  
   162  ### Containers
   163  
   164  Running containers are mounted below `/var/lib/docker/aufs/mnt/<container-id>`.
   165   This is where the AUFS union mount point that exposes the container and all
   166  underlying image layers as a single unified view exists. If a container is not
   167  running, it still has a directory here but it is empty. This is because AUFS
   168  only mounts a container when it is running. With Docker 1.10 and higher,
   169   container IDs no longer correspond to directory names under
   170  `/var/lib/docker/aufs/mnt/<container-id>`.
   171  
   172  Container metadata and various config files that are placed into the running
   173  container are stored in `/var/lib/docker/containers/<container-id>`. Files in
   174  this directory exist for all containers on the system, including ones that are
   175  stopped. However, when a container is running the container's log files are
   176  also in this directory.
   177  
   178  A container's thin writable layer is stored in a directory under
   179  `/var/lib/docker/aufs/diff/`. With Docker 1.10 and higher, container IDs no
   180  longer correspond to directory names. However, the containers thin writable
   181  layer still exists under here and is stacked by AUFS as the top writable layer
   182  and is where all changes to the container are stored. The directory exists even
   183   if the container is stopped. This means that restarting a container will not
   184  lose changes made to it. Once a container is deleted, it's thin writable layer
   185  in this directory is deleted.
   186  
   187  ## AUFS and Docker performance
   188  
   189  To summarize some of the performance related aspects already mentioned:
   190  
   191  - The AUFS storage driver is a good choice for PaaS and other similar use-cases
   192   where container density is important. This is because AUFS efficiently shares
   193  images between multiple running containers, enabling fast container start times
   194   and minimal use of disk space.
   195  
   196  - The underlying mechanics of how AUFS shares files between image layers and
   197  containers uses the systems page cache very efficiently.
   198  
   199  - The AUFS storage driver can introduce significant latencies into container
   200  write performance. This is because the first time a container writes to any
   201  file, the file has be located and copied into the containers top writable
   202  layer. These latencies increase and are compounded when these files exist below
   203   many image layers and the files themselves are large.
   204  
   205  One final point. Data volumes provide the best and most predictable
   206  performance. This is because they bypass the storage driver and do not incur 
   207  any of the potential overheads introduced by thin provisioning and
   208  copy-on-write. For this reason, you may want to place heavy write workloads on
   209  data volumes.
   210  
   211  ## Related information
   212  
   213  * [Understand images, containers, and storage drivers](imagesandcontainers.md)
   214  * [Select a storage driver](selectadriver.md)
   215  * [Btrfs storage driver in practice](btrfs-driver.md)
   216  * [Device Mapper storage driver in practice](device-mapper-driver.md)