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115 lines
4.2 KiB
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:title: Kernel Requirements
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:description: Kernel supports
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:keywords: kernel requirements, kernel support, docker, installation, cgroups, namespaces
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.. _kernel:
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Kernel Requirements
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===================
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In short, Docker has the following kernel requirements:
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- Linux version 3.8 or above.
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- `AUFS support <http://aufs.sourceforge.net/>`_.
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- Cgroups and namespaces must be enabled.
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The officially supported kernel is the one recommended by the
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:ref:`ubuntu_linux` installation path. It is the one that most developers
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will use, and the one that receives the most attention from the core
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contributors. If you decide to go with a different kernel and hit a bug,
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please try to reproduce it with the official kernels first.
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If you cannot or do not want to use the "official" kernels,
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here is some technical background about the features (both optional and
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mandatory) that docker needs to run successfully.
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Linux version 3.8 or above
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--------------------------
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Kernel versions 3.2 to 3.5 are not stable when used with docker.
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In some circumstances, you will experience kernel "oopses", or even crashes.
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The symptoms include:
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- a container being killed in the middle of an operation (e.g. an ``apt-get``
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command doesn't complete);
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- kernel messages including mentioning calls to ``mntput`` or
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``d_hash_and_lookup``;
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- kernel crash causing the machine to freeze for a few minutes, or even
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completely.
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While it is still possible to use older kernels for development, it is
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really not advised to do so.
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Docker checks the kernel version when it starts, and emits a warning if it
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detects something older than 3.8.
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See issue `#407 <https://github.com/dotcloud/docker/issues/407>`_ for details.
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AUFS support
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------------
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Docker currently relies on AUFS, an unioning filesystem.
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While AUFS is included in the kernels built by the Debian and Ubuntu
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distributions, is not part of the standard kernel. This means that if
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you decide to roll your own kernel, you will have to patch your
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kernel tree to add AUFS. The process is documented on
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`AUFS webpage <http://aufs.sourceforge.net/>`_.
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Cgroups and namespaces
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----------------------
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You need to enable namespaces and cgroups, to the extend of what is needed
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to run LXC containers. Technically, while namespaces have been introduced
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in the early 2.6 kernels, we do not advise to try any kernel before 2.6.32
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to run LXC containers. Note that 2.6.32 has some documented issues regarding
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network namespace setup and teardown; those issues are not a risk if you
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run containers in a private environment, but can lead to denial-of-service
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attacks if you want to run untrusted code in your containers. For more details,
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see `[LP#720095 <https://bugs.launchpad.net/ubuntu/+source/linux/+bug/720095>`_.
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Kernels 2.6.38, and every version since 3.2, have been deployed successfully
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to run containerized production workloads. Feature-wise, there is no huge
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improvement between 2.6.38 and up to 3.6 (as far as docker is concerned!).
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Extra Cgroup Controllers
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------------------------
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Most control groups can be enabled or disabled individually. For instance,
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you can decide that you do not want to compile support for the CPU or memory
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controller. In some cases, the feature can be enabled or disabled at boot
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time. It is worth mentioning that some distributions (like Debian) disable
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"expensive" features, like the memory controller, because they can have
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a significant performance impact.
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In the specific case of the memory cgroup, docker will detect if the cgroup
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is available or not. If it's not, it will print a warning, and it won't
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use the feature. If you want to enable that feature -- read on!
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Memory and Swap Accounting on Debian/Ubuntu
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-------------------------------------------
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If you use Debian or Ubuntu kernels, and want to enable memory and swap
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accounting, you must add the following command-line parameters to your kernel::
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cgroup_enable=memory swapaccount
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On Debian or Ubuntu systems, if you use the default GRUB bootloader, you can
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add those parameters by editing ``/etc/default/grub`` and extending
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``GRUB_CMDLINE_LINUX``. Look for the following line::
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GRUB_CMDLINE_LINUX=""
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And replace it by the following one::
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GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount"
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Then run ``update-grub``, and reboot.
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