Namespace, CGroup, and Union file-system are the basic building blocks of a container. Let’s have our focus on file-system. Why yet another file-system for the container? Is Conventional Linux file-systems like ext2, ext3, ext4, XFS, etc. not good enough to meet the purpose? In this blog post, I will try to answer these questions. Here we will be delving deeply into the Union File System and a few of its essential properties.

1. Presented By:
Mayank Bairagi
Sr. Lead Consultant
Union FileSystem
- A Building Blocks of a Container

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Building Blocks of a Container
Union Capable File System
Copy on Write (CoR)
Demo
References

3. Containers

4. A Misconception
● Containers don't run on container runtime (docker -> dockerd, containerd, runC). Docker is
a daemon which makes container easy to run. Container is a abstraction defined at higher
level. Low level details are already a part of linux kernel.

5. Building blocks of a Container

6. Fatty Big Container is a problem.
● Slow Down Deployment

7. Fatty Big Container is a problem.
● Slow Down Deployment
● More Disk Space

8. Fatty Big Container is a problem.
● Slow Down Deployment
● More Disk Space
● Increase cost of object storage

9. Fatty Big Container is a problem.
● Slow Down Deployment
● More Disk Space
● Increase cost of object storage
● Decrease I/O performance

10. Fatty Big Container is a problem.
● Slow Down Deployment
● More Disk Space
● Increase cost of object storage
● Decrease I/O performance
● Harder to Audit

11. Fatty Big Container is a problem.
● Slow Down Deployment - Network
● More Disk Space - Storage
● Increase cost of object storage - Budget
● Decrease I/O performance - Performance
● Harder to Audit - Security

12. Layered Architecture
A container is composed of multiple branches. In docker’s terminology, branches are also
known as layers. A sandbox of a container is composed of one or more image layers and a
container layer. Container layer is writable, image layers are read-only.

13. Layered File Structure

14. Union File System
"Union file system works on top of the other file-systems. It gives a single coherent and
unified view to files and directories of separate file-system. In other words, it mounts multiple
directories to a single root. It is more of a mounting mechanism than a file system."

16. Properties of a Union File System
1. Logical merge of multiple layers.
2. Read-only lower layers, writable upper layer.
3. Start reading from the upper layer than defaults to lower layers.
4. Copy on Write (CoW)
5. Simulate removal from lower directory through whiteout file

17. Copy on Write
"Copy-on-write is a similar strategy of sharing and copying, in which the system processes
that need access to the same data share the same instance of that data rather than having
their own copy. At some point, if any one process wants to modify or write to the data, only
then does the operating system make a copy of the data for that process to use. Only the
process that needs to write has access to the data copy. All other processes continue to use
the original data."

18. Why yet another file-
system, when mount is
available

19. A Sample Use Case

20. Create sample directory structure and virtual partitions.

21. Experiment 1 : Mount multiple directories on a common mount point using
ext* file-systems.

22. Experiment 2: Mount multiple directories on a common mount point using
Union File System (OverlayFS).

23. Experiment 3: Demonstrate Copy-on-write (CoR)

24. Experiment 3: Demonstrate Copy-on-write (CoR)

25. Experiment 4: Deleting files from “lower” layer.

26. Thank You !
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