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+.. _kernel:
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+
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+Kernel Requirements
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+===================
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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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+
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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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+
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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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+
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+
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+Namespaces and Cgroups
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+----------------------
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+
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+Important Note About Pre-3.8 Kernels
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+------------------------------------
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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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+
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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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+
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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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+
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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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+
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+See issue `#407 <https://github.com/dotcloud/docker/issues/407>`_ for details.
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+
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+
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+Extra Cgroup Controllers
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+------------------------
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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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+
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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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+
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+
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+Memory and Swap Accounting on Debian/Ubuntu
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+-------------------------------------------
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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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+
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+ cgroup_enable=memory swapaccount
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+
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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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+
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+ GRUB_CMDLINE_LINUX=""
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+
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+And replace it by the following one::
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+
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+ GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount"
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+
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+Then run ``update-grub``, and reboot.
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+
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+
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+AUFS
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+----
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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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+
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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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+
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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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+
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+
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+BTRFS, ZFS, OverlayFS...
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+------------------------
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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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+
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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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+
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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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