The document summarizes a talk given at the Linux Plumbers Conference 2014 about Docker and the Linux kernel. It discusses what Docker is, how it uses kernel features like namespaces and cgroups, its different storage drivers and their issues, kernel requirements, and how Docker and kernel developers can collaborate to test and improve the kernel and Docker software.

1. Linux Plumbers Conference 2014, Düsseldorf
Docker and the Linux kernel
Cristian S., Docker Inc.
cristian@docker.com

2. What is Docker ?

3. The Matrix From Hell

4. Another Matrix From Hell

5. Solution:
the intermodal shipping container

6. Solved!

7. Solution to the deployment problem:
the Linux container

8. Solved!

9. High level overview
• Uses namespaces & cgroups
• Runs on mainline kernels
• Lower overhead than VMs
• Can run a full system with an init or a
single service/process
• Snapshot based approach to build
one image on top of another

10. Docker's code
• Licensed under the Apache 2 license
• There's no paid/premium/commercial
version
• Docker and its code are free and will
be free
• Lives at:
https://github.com/docker/docker/

11. Use cases
• Application development,
testing, packaging & deployment
• PaaS/SaaS/cloud infrastructure
• Application & service isolation
• Stress testing & benchmarking
(including the Linux kernel)

12. Under the hood
• Exec drivers provide the execution environment
(virtualization/container tech)
– native (libcontainer based) and LXC
– are platform dependent
– native is the default
• Graph drivers are the storage providers
– aufs, devicemapper, btrfs and vfs
– vfs shouldn't be used (only used by the tests by
default)
– PRs open on GitHub for ZFS and OverlayFS
• Existing graph & exec drivers only support Linux

13. Kernel requirements
• Kernel 3.8 is the absolute minimum
(except RHEL's 2.6.32)
• Stable & supported kernels >= 3.10 are
recommended
• BTRFS has special requirements
• Kernel configuration can be checked using
https://raw.githubusercontent.com/docker/
docker/master/contrib/check-config.sh

14. AUFS graph driver stability &
performance
• Operations with containers are faster than devicemapper &
(sometimes) btrfs
• Known problems
– Stale NFS file handle, cap_set_file, invalid argument on mount
– Causes troubles on btrfs, remote file systems and many file
systems which aren't ext3/ext4
– Direct IO problems & poor performance for IO intensive workloads
• Limitations
– Requires aufs-tools for auplink to dereference hard links
– No support for hard links across layers
– Limit of 127 layers
– Can't be used on Fedora/RHEL/all systems which don't apply AUFS
patches and don't ship aufs-tools
• AUFS is developed outside of the mainline kernel tree
• Update the kernel using distro updates

15. Devicemapper graph driver stability
& performance
• Uses loopback mounted block devices by default
• Allows EXT4 or XFS to be used
• Known problems
– EBUSY errors (fix to be tested)
– Potential file system corruption bug with ext4
● Might be caused by loopback mounted block devices
– Older kernels: space not freed when freed on the file
system, various kernel errors and problems
• Limitations
– Devicemapper requires configuration of the storage to avoid
the use of loopback mounted block devices
• Update the kernel using distro updates

16. btrfs graph driver stability &
performance
• BTRFS bugs are a problem for Docker
• Using the RAID like features of BTRFS is likely to cause data loss
• Known problems
– Data can be corrupted if exotic mount options are used
– Kernel 3.8 & older than the latest stable kernels can cause data loss &
corruption
– The file system becomes slower the more data is written & stored on it
– Performance degrades quickly & fragmentation is a problem
– Balancing the file system to fix fragmentation could trigger some bugs
• BTRFS is used automatically if Docker's root folder is on BTRFS
• Update the kernel using distro updates
• Using the latest minor version of supported kernels is recommended

17. Performance & stability
• Generally speaking, the kernel has been stable
– Fixes have been made to file systems, namespaces, cgroups,
netfilter, aufs, btrfs, devicemapper and other kernel components
– Recent kernels and the newest minor versions of LTS kernels have
fixed a lot of problems, including devicemapper and btrfs problems
– Some PID 1 issues are still being discussed
– Changes around the cgroups and the namespaces have been
made; this has also helped improve stability for containers
• Performance is actively being studied
– Docker's code has been improved to use less memory & be faster
– Performance needs to be studied on the kernel side to achieve
better scalability by a) making fewer syscalls in Docker where
possible, b) making those syscalls faster in the kernel

18. Kernel recommendations
• Avoid running kernels no longer supported
by your Linux distribution (kernel 3.8 lts-raring
from Ubuntu 12.04.x)
• Open Docker issues on github; post errors
(full kernel panics, btrfs check output,
Docker daemon logs, `docker info`,
`docker version` and `uname -a` output)
when encountering errors
• File bug reports with your distribution

19. What can Docker do for the kernel?
• Can be used to test the kernel's stability and
evaluate its performance
• Changes made to the kernel can be tested
easily against Docker
• Can be used to test the running kernel to
avoid breaking user space compatibility
• Makes it simple to mix workloads for stress
testing and performance testing
• Exposes some hard to trigger kernel problems

20. What can a kernel developer do
with Docker?
• Find bottlenecks in the kernel's code
• Run a KVM VM in a container
• Stress testing for file systems, network,
namespaces and the kernel in general
• Large scale testing with containers
• Hardware testing in containers
• Network testing with advanced
topologies

21. How can someone contribute to Docker?
What about kernel developers?
• You don't necessarily have to write code
• Providing feedback, doing code review or
telling the right kernel developer to do
that are also useful helpful
• Suggestions on how to debug specific
kernel bugs and bugs in general are
welcome
• Contributing to Docker is also helping the
kernel because Docker relies on the kernel

22. Thank you! Questions?
http://docker.com
@docker