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153 lines
6.2 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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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 for some reason 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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In short, you need kernel version 3.8 (or above), compiled to include
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`AUFS support <http://aufs.sourceforge.net/>`_. Of course, you need to
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enable cgroups and namespaces.
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Namespaces and Cgroups
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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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Starting with version 3.7, the kernel has basic support for
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`Checkpoint/Restore In Userspace <http://criu.org/>`_, which is not used by
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docker at this point, but allows to suspend the state of a container to
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disk and resume it later.
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Version 3.8 provides improvements in stability, which are deemed necessary
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for the operation of docker. Versions 3.2 to 3.5 have been shown to
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exhibit a reproducible bug (for more details, see issue
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`#407 <https://github.com/dotcloud/docker/issues/407>`_).
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Version 3.8 also brings better support for the
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`setns() syscall <http://lwn.net/Articles/531381/>`_ -- but this should not
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be a concern since docker does not leverage on this feature for now.
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If you want a technical overview about those concepts, you might
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want to check those articles on dotCloud's blog:
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`about namespaces <http://blog.dotcloud.com/under-the-hood-linux-kernels-on-dotcloud-part>`_
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and `about cgroups <http://blog.dotcloud.com/kernel-secrets-from-the-paas-garage-part-24-c>`_.
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Important Note About Pre-3.8 Kernels
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------------------------------------
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As mentioned above, kernels before 3.8 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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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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AUFS
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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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Note: the AUFS patch is fairly intrusive, but for the record, people have
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successfully applied GRSEC and AUFS together, to obtain hardened production
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kernels.
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If you want more information about that topic, there is an
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`article about AUFS on dotCloud's blog
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<http://blog.dotcloud.com/kernel-secrets-from-the-paas-garage-part-34-a>`_.
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BTRFS, ZFS, OverlayFS...
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------------------------
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There is ongoing development on docker, to implement support for
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`BTRFS <http://en.wikipedia.org/wiki/Btrfs>`_
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(see github issue `#443 <https://github.com/dotcloud/docker/issues/443>`_).
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People have also showed interest for `ZFS <http://en.wikipedia.org/wiki/ZFS>`_
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(using e.g. `ZFS-on-Linux <http://zfsonlinux.org/>`_) and OverlayFS.
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The latter is functionally close to AUFS, and it might end up being included
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in the stock kernel; so it's a strong candidate!
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Would you like to `contribute
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<https://github.com/dotcloud/docker/blob/master/CONTRIBUTING.md>`_
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support for your favorite filesystem?
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